Hello Jeremy603,

Not sure this is going to solve your issue, but worth a try if you didn't already do it: - open History: Ctrl+Shift+H - identify the link that is throwing a 404 error, right click it and select "Forget about this site"

Any luck?

Regards,

Great suggestion and I just learned something new, so thank you.

But, unfortunately, no luck.

I'm not opposed to wiping every last trace of Firefox from my system and starting over. But there are two reasons why I haven't taken that drastic step, yet: One, it's a pain in the butt and, two, I like to learn so I really want to figure out what is causing this issue.

UPDATE:

I think this problem originates with a failed login attempt on the affected pages.

I use Zenfolio for my photography business. Never had a problem until today when I had a total brain failure and had to request a password reset from the site. Once I received the email with my new password, I tried to log on and, just like the other pages mentioned above, I couldn't get through. Only, with Zenfolio, instead of a 404 or 403 error page, their internal system just states right on the splash screen that an error occurred. Now, no matter what I do, I can't log into my account.

So I went to another browser....and logged on with no problem! I do know from memory that I have botched log-on attempts on each of the sites mentioned in my original question. So hopefully this new information can help somebody to help me. *fingers crossed*

Thanks, again!

Please post all the links you are having problems with.

Also, try this out;

Add WWW to the address.

Add http:// or https:// to the address.

If these are present already, try without https:// or WWW.

Hi, Fred.

Thank you for replying! The problem is with login re-directs, so modifying the URL header does nothing to address the issue. In other words, the problem comes AFTER you press the "Submit" or "Login" or whatever other button a site uses to send the login credentials to the host database for verification.

Let's stick with the three sites I've already mentioned as examples, for now:

1. https://accounts.craigslist.org/login?rt=L&rp=/login/home

  *which re-directs upon login to*
    https://accounts.craigslist.org/login 
  *which displays their 404 error page.*

2. https://sis.mccnh.edu/manc/twbkwbis.P_WWWLogin

  *which re-directs upon login to*
   https://sis.mccnh.edu/manc/twbkwbis.P_ValLogin
  *which displays their 403 Forbidden page.*

3. https://secure.zenfolio.com/603mediagroup/login.html

  *which does NOT re-direct upon log-in but, rather, stays on the splash screen and displays an error message: "An unexpected error has occurred. Please try again later. If this problem persists, contact Support."*

Hope this helps! Thank you!

Try to set a lower value to the network.http.max-connections pref on the about:config page.

• network.http.max-connections = 32

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

No dice, Cor-el.  :(

I have a feeling I'm just going to end up obliterating Firefox completely from my laptop which I really don't want to do. I'd honestly like to find an answer to this very strange problem rather than just defaulting to an un-install/re-install as a solution....

Please continue with ideas if y'all have any and know that I'm super appreciative of your efforts!

Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache

Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

and

• Remove Cookies

Warning ! ! This will log you out of all sites you're logged in to. Press the Alt or F10 key to bring up the tool bar. Followed by;

Windows; Tools > Options Linux; Edit > Preferences Mac; application name > Preferences

Then Privacy. Under History, select Firefox will Use Custom Settings. There is a button on the right side called Show Cookies.

If there is still a problem, Start Firefox in Safe Mode {web link} While you are in safe mode; Press the Alt or F10 key to bring up the tool bar. Followed by;

Windows; Tools > Options Linux; Edit > Preferences Mac; application name > Preferences

Then Advanced > General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Then restart.

Thanks Fred but, as mentioned in my original post, all of that has been done, already.  :(

Can you post your troubleshooting information?

• https://support.mozilla.org/kb/Using+the+Troubleshooting+Information+page

You can check the connection settings.

• Tools > Options > Advanced > Network : Connection > Settings
• https://support.mozilla.org/kb/Options+window+-+Advanced+panel

If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/kb/Firefox+cannot+load+websites+but+other+programs+can

If clearing cookies didn't help then it is possible that the cookies.sqlite file that stores the cookies got corrupted.

• Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete other present cookies files like cookies.sqlite-journal in the Firefox profile folder in case the file cookies.sqlite got corrupted.

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

Do a malware check with several malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

Cor-el, as mentioned to Fred, all of that stuff has already been tried - and stated in my original question. I know it's just a copy-and-paste answer, but I covered all of that in my original question. Thank you, though!

Here is the info from the troubleshooting page (please note before supplying copy-and-paste answers that I've already tried disabling extensions, add-ons, etc.) :

Application Basics

Name: Firefox Version: 31.0 User Agent: Mozilla/5.0 (Windows NT 6.2; WOW64; rv:31.0) Gecko/20100101 Firefox/31.0

Crash Reports for the Last 3 Days

All Crash Reports (including 3 pending crashes in the given time range)

Extensions

Name: Adblock Plus Version: 2.6.4 Enabled: true ID: {d10d0bf8-f5b5-c8b4-a8b2-2b9879e08c5d}

Name: DownThemAll! Version: 2.0.17 Enabled: true ID: {DDC359D1-844A-42a7-9AA1-88A850A938A8}

Name: Password Exporter Version: 1.2.1 Enabled: true ID: {B17C1C5A-04B1-11DB-9804-B622A1EF5492}

Name: SeoQuake Version: 2.8.14 Enabled: true ID: {317B5128-0B0B-49b2-B2DB-1E7560E16C74}

Name: Adobe Acrobat - Create PDF Version: 1.2 Enabled: false ID: web2pdfextension@web2pdf.adobedotcom

Name: Facebook Photo Appraiser Version: 1.1 Enabled: false ID: jid1-MYUA6noZreQonw@jetpack

Graphics

Adapter Description: AMD Radeon HD 7500G Adapter Drivers: aticfx64 aticfx64 aticfx64 aticfx32 aticfx32 aticfx32 atiumd64 atidxx64 atidxx64 atiumdag atidxx32 atidxx32 atiumdva atiumd6a atitmm64 Adapter RAM: 512 Device ID: 0x990a Direct2D Enabled: true DirectWrite Enabled: true (6.2.9200.16581) Driver Date: 10-18-2012 Driver Version: 9.2.1.2000 GPU #2 Active: false GPU Accelerated Windows: 1/1 Direct3D 10 Vendor ID: 0x1002 WebGL Renderer: Google Inc. -- ANGLE (AMD Radeon HD 7500G Direct3D9Ex vs_3_0 ps_3_0) windowLayerManagerRemote: false AzureCanvasBackend: direct2d AzureContentBackend: direct2d AzureFallbackCanvasBackend: cairo AzureSkiaAccelerated: 0

Important Modified Preferences

accessibility.typeaheadfind.flashBar: 0 browser.cache.disk.capacity: 358400 browser.cache.disk.smart_size_cached_value: 358400 browser.cache.disk.smart_size.first_run: false browser.cache.disk.smart_size.use_old_max: false browser.cache.frecency_experiment: 1 browser.places.smartBookmarksVersion: 7 browser.sessionstore.upgradeBackup.latestBuildID: 20140716183446 browser.startup.homepage_override.buildID: 20140716183446 browser.startup.homepage_override.mstone: 31.0 browser.tabs.loadInBackground: false dom.mozApps.used: true extensions.lastAppVersion: 31.0 font.internaluseonly.changed: true gfx.direct3d.last_used_feature_level_idx: 0 network.cookie.prefsMigrated: true network.dns.disablePrefetch: true network.http.max-connections: 32 network.http.sendRefererHeader: 0 places.database.lastMaintenance: 1409233909 places.history.expiration.transient_current_max_pages: 104858 plugin.disable_full_page_plugin_for_types: application/pdf plugin.importedState: true privacy.cpd.formdata: false privacy.cpd.offlineApps: true privacy.cpd.sessions: false privacy.cpd.siteSettings: true privacy.donottrackheader.enabled: true privacy.sanitize.migrateFx3Prefs: true privacy.sanitize.timeSpan: 0 storage.vacuum.last.index: 1 storage.vacuum.last.places.sqlite: 1408775389

user.js Preferences

Your profile folder contains a user.js file, which includes preferences that were not created by Firefox.

JavaScript

Incremental GC: true

Accessibility

Activated: false Prevent Accessibility: 0

Library Versions

NSPR Expected minimum version: 4.10.6 Version in use: 4.10.6

NSS Expected minimum version: 3.16.2 Basic ECC Version in use: 3.16.2 Basic ECC

NSSSMIME Expected minimum version: 3.16.2 Basic ECC Version in use: 3.16.2 Basic ECC

NSSSSL Expected minimum version: 3.16.2 Basic ECC Version in use: 3.16.2 Basic ECC

NSSUTIL Expected minimum version: 3.16.2 Version in use: 3.16.2

Experimental Features

You are blocking the referrer.

• network.http.sendRefererHeader: 0

Not all web servers like this.

See also:

• http://kb.mozillazine.org/network.http.sendRefererHeader

BINGO!!!!

Changing the network.http.sendRefererHeader value to "1" instead of "0" fixed the problem.

Thank you so much! Now I'm going to spend some time learning about this function to figure out how it works.

Oh, for the record, I have never messed with that stuff, so I have no idea why it would have changed....

Thank you, again, for all your help!  :)

Did you recently update or use the Firefox full installer?

Bug Report 1031347 – Firefox Full Installer Changes User Settings

I did, yes. As mentioned in my original question, I deleted Firefox using iObit Uninstall, shredded the files (except the profile folder, because I wasn't ready to do that step, yet), and downloaded Firefox directly from the Mozilla site through Google Chrome. I then used the Installer to set it up.

What's interesting about the page you posted is that the OP was not able to connect to ANY websites because of a change in his proxy settings. That was not my issue, at all. Mine was a handful of login re-direct pages that would error out and it appears to be fully centered around that "referer" trigger value being set to "0". Makes me wonder how many people have battled the same issue who are less tech-savvy and less able to understand how or where to look for help....

Thank you, again, for all the help! My problem is definitely solved as I am having no difficulties with logging on to those websites mentioned here or any others.