This can possibly happen when cookies are blocked or corrupted.

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache":

• Firefox/Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Firefox/Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies"

If clearing cookies didn't help then It is possible that the cookies.sqlite file in the Firefox profile folder that stores the cookies got corrupted.

• rename/remove cookies.sqlite (cookies.sqlite.old) and if present delete cookies.sqlite-shm and cookies.sqlite-wal in the Firefox profile folder in case cookies.sqlite got corrupted.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

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

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks and history and cookies and passwords and cache and exceptions, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.

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

See "Creating a profile":

• https://support.mozilla.org/en-US/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

Thank you for your kind reply! I did all of those things. I deleted the old profile by "firefox.exe -P" and afterwards created a new profile by which I was initially able to log into the router, when starting Firefox with "firefox -P". Apparently, this disables certain functions. However, when I started Firefox a subsequent next time with the new, clean profile, the exact same issue in question returned, i.e. I could no longer log into the router.

Before I start Firefox a second time, if I delete the prefs.js file in my profile, then I can log in again, but only till next time. So something may be corrupting that file?

Hi, this is what Safe Mode disables http://kb.mozillazine.org/Safe_mode

You could try typing about:config into the address bar, press Enter, accept the warning, scroll down to javascript.options.baselinejit and see if it's set to default - which is True. You can change it if necessary, by double clicking on it.

Thank you very much for your concern. After installing Malwarebytes and performing a scan, this issue ceased to happen. :-)

http://www.malwarebytes.org/mbam.php

I hope that it does not return any more. :-)

There are more ways of getting rid of malware in this thread Here, the moderator Cor-el (above) gave the following answer: "Do a malware check with some 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: Home Page:

   http://www.microsoft.com/windows/products/winfamily/defender/default.mspx

- 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 "