Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

https://support.mozilla.org/en-US/kb/websites-look-wrong-or-appear-differently

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

https://support.mozilla.org/en-US/kb/websites-look-wrong-or-appear-differently

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

Thank you for your reply. Starting Firefox in safe mode doesn't change anything. I tried to follow the troubleshooting instructions, but I can't get "preferences" - from the Firefox menu - except under Tools and Options, and when I click on "Help" nothing happens. One of the instructions is to go to "Advanced" and click on the "Encryption" tab, and then check to make sure the two relevant SSL's are checked. However, since the "tune-up" that Firefox did, I no longer have an encryption tab. Now I can't even get the instruction page back. This is very frustrating, because, while I had issues with Firefox being very slow and not responding often (which is why I did the tune-up), now things are worse, and there are so many pages that I can't open because the "secure connection failed." The pages I'm trying to get to aren't even secure pages, and I have no problem opening them on Edge and IE. Since the tune-up, I can't even bring up things that I was able to bring up before, including something as important as "Help."

Try to rename the cert8.db file (cert8.db.old) and delete the cert_override.txt file in the Firefox profile folder to remove intermediate certificates and exceptions that Firefox has stored.

If that has helped to solve the problem then you can remove the renamed cert8.db.old file. Otherwise you can rename (or copy) the cert8.db.old file to cert8.db to restore the previously stored intermediate certificates. Firefox will automatically store intermediate certificates when you visit websites that send such a certificate.

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

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

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

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) to see if that helps.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Thanks again, but I can't make heads or tails of some of these suggestions. Since the "tune-up" from Mozilla, my "Options" don't have some of the choices that you're referring to. There's nowhere for me to look at certificates, for example. I continue to be bedeviled with "Firefox Not Responding" and very slow browsing, but I can't even get to the point of addressing those issues, since I'm met with a barrage of "secure connection failed" with some nonsense about certificates. I say nonsense, because not a single actual secure page/site that I try to access is a problem. It's part of a page that pops up while the rest of the page is fine, or, as I said in the first place, I can no longer log onto msn.com through my hotmail account, even though it should automatically come up when I am logged into hotmail (or should easily allow me to login anyway). I just paid a half dozen bills and checked my bank account information, and had no problem with any of those websites. I have not added/removed/changed any software/hardware since upgrading to Windows 10, and these "Firefox Not Responding" and slow browsing predate that. The "secure connection failed" has only happened in the past few days since I did the Mozilla-recommended "tune-up." I don't want to have to delete Firefox and start anew - I don't want to re-enter all bookmarks and saved passwords, etc.

You can open the Advanced tab in Options/Preferences to find the certificate

• about:preferences#advanced

There is security software like Avast and Kaspersky and BitDefender and ESET that intercept secure connections and send their own certificate.

If you can't inspect the certificate via "I Understand the Risks" then try this:

Open the "Add Security Exception" window by pasting this chrome URL in the Firefox location/address bar and check the certificate:

• chrome://pippki/content/exceptionDialog.xul

In the location field of this window type or paste the URL of the website.

• retrieve the certificate via the "Get certificate" button
• click the "View..." button to inspect the certificate in the Certificate Viewer

You can inspect details like the issuer and the certificate chain in the Details tab of the Certificate Viewer. Check who is the issuer of the certificate. If necessary then you can attach a screenshot that shows the certificate viewer.

I appreciate the replies, but unfortunately my problem hasn't been replied. I have followed the various instructions, but I can't find the things you are referring to, such as certificates information in options>advanced. I finally gave up and uninstalled Firefox. When I tried again, somehow the settings were the same, favorites and saved passwords were all there, and I still get the same "secure connection failed" error message when trying to access non-secure sites. I have no problem using truly secure sites, such as when I access bank and credit card accounts. I also looked in the Registry, and I still see listings for Firefox, but I'm hesitant to remove them. It certainly doesn't seem that they should still be there when I have used a powerful uninstall that is supposed to remove every trace of the program. The one odd thing is that it has deleted the plug-in for a program that I have installed on the computer, but didn't delete 2 old plug-ins that don't belong there since newer versions have been installed (I tried to uninstall the older versions). Any suggestions on how I can remove all traces of Firefox so that I can start with a fresh install?

Create a new profile as a test to check if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

See also:

• https://support.mozilla.org/kb/refresh-firefox-reset-add-ons-and-settings

Firefox comes in two or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And one folder in the profile of each user on the computer.

If you remove the Firefox folder, the user folders would not be affected.