This issue can be caused by corrupted cookies.

Clear the cache and the cookies from sites that cause problems.
"Clear the Cache":

• Firefox > Preferences > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove the Cookies" from sites causing problems:

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

Does not solve the problem.

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

Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete cookies.sqlite-journal and cookies.txt, if they exist, in the Profile Folder in case the file cookies.sqlite got corrupted.

You can also do a test by switching on Private Browsing.
Firefox doesn't use the disk cache and the currently saved cookies if you are in PB mode.

• To see all History and Cookie settings, choose: Tools > Options > Privacy, choose the setting Firefox will: Use custom settings for history
• Check-mark: [X] "Permanent Private Browsing mode"

Hi everybody, I am experiencing the same problem, and unfortunately clearing the cache and the cookies didn't help. Also, in my Profile Folder there is only one file called ri4ebjmt.default (i.e., no cookies.*). So, what should I do now?

Any help is appreciated.

The folder ri4ebjmt.default is the Firefox profile folder.
You need to look in that folder to see the files in it.

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder