Click Start > Run and type in
%appdata%\Mozilla\Firefox\ and click OK. Delete all stuff that's there
as soon as you have read what's below. This will erase all bookmarks and stuff that you saved/modified, so just like a fresh install.
You can
save some settings (e.g. bookmarks) by diving into the Profile folder and the folder with the weird name. The files bookmarks.html and prefs.js might be worth saving. After deleting the profile and restarting Firefox you have to start all over again. Then close Firefox, go to the Profiles\weird name folder again and replace the newly made bookmarks.html and prefs.js with the saved files.
Note that one of the files is corrupt, but I don't know which one. If you save and put back the corrupt file you'll have the same problem all over again. I don't think the bookmarks file and the preferences file are corrupt though.
Good luck
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