I was having an issue recently that none of my settings/preferences were saved in Firefox. It opens the Firefox updated page along with extensions updated page every time I open the browser. The manual override of about:blank is also not reflecting.
Solution is just delete your pref.js from your profile folder or make sure that it is not read only. Interestingly my pref.js was read only I just deleted it, I believe making it writable will also fix the issue.
Profile folder location(s)
Windows NT (NT4.x/2000/XP/Vista/7): "%APPDATA%\Mozilla\"
Unix/Linux : ~/.mozilla/
Mac OS X: ~/Library/Mozilla/ or ~/Library/Application Support/