See these articles for some suggestions:

• https://support.mozilla.com/kb/Firefox+has+just+updated+tab+shows+each+time+you+start+Firefox
• https://support.mozilla.com/kb/How+to+set+the+home+page - Firefox supports multiple home pages separated by '|' symbols

• https://support.mozilla.com/kb/Preferences+are+not+saved
• http://kb.mozillazine.org/Preferences_not_saved

Unfortunately, none of that solved the problem. I had to disable two extensions that were recently updated.

Did you try to delete the file prefs.js in the Firefox Profile Folder?

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

I found several prefs.js files, I deleted them (but not a small one that seemed to be a default setting file). Since I had Firefox open at the time of deletion, it wrote one of the files again when I close FF. Seeing that, and also checking that the files were not write protected (before the deletion, of course), I assumed that there were not problems with those files.

FF was still with the problem after that.

A time later, I opened Thunderbird to go to a newsgroup, and I found that it has lost all it's configuration. Fortunately, I still had the files in the Recicle Bin, so I restored them.

You only need to remove the file in the Firefox Profile Folder that you can reach via the "Help > Troubleshooting Information > Profile Directory: Open Containing Folder" link.

There is no need to remove that file if present in another location like you did with the file in the Thunderbird profile.

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

Hello, I found the profile folder. I tried deleting the prefs.js but that didn't fix the problem. Then I found that there were several *.js files there, and the problem was fixed deleting the user.js file. I don't see much other change (so far), I even had the password to this site still stored in FF, and also all the bookmarks.

Thanks cor-el.