Clear the cache and cookies only from websites that cause problems.

"Clear the Cache":

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

"Remove Cookies" from sites causing problems:

• Firefox > Preferences > Privacy > Cookies: "Show Cookies"

You may have to allow third-party cookies (visited or all).

To see all History and Cookie settings, choose:

• Firefox > Preferences > Privacy > Firefox will: "Use custom settings for history"

Thanks for your reply. Clearing the cache and cookies does not fix the problem. I had already tried that in the troubleshooting steps. Thanks

Are there any special characters in the link that may be causing this issue?

If you use a bookmark then try to navigate to this page starting with the log in page of the site.

It is possible that the cookies.sqlite file that stores the cookies is corrupted if clearing cookies doesn't work.

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 Firefox profile folder:

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

• http://kb.mozillazine.org/Cookies
• https://support.mozilla.org/kb/Deleting+cookies

Your "More System Details" shows that you block "third party" cookies. Try allowing third party cookies as a test to see whether that solves the problem.

Firefox menu > Preferences > Privacy

With Firefox will: Use custom settings for privacy selected, you can change "Accept third party cookies" to Always.

See also: Websites say cookies are blocked - Unblock them

Note that I posted this already above, but failed to verify if it has been tried.

You may have to allow third-party cookies (visited or all).
To see all History and Cookie settings, choose:

• Firefox > Preferences > Privacy > Firefox will: "Use custom settings for history"

Bingo! It's working. I unselected the Always use private browsing mode and it fixed the problem. I already had the other entries selected and Accept third-part cookies: set to Always. and Keep until: they expire. The only entry not selected was: Clear history when Firefox closes. Thanks a lot for your help and quick response. Don Smith