By Resetting firefox this can be solve however it might caused for specific preferences not saved or at all. As it fix a wide range of errors. But after resetting firefox you might lose some of stuffs from your firefox (history, bookmark, cookies etc) so best to make a backup before this. You can also fix it by Checking if any Third Party software is not conficting with the settings (for example McAfee Privacy Service,Norton 360 etc...) Also By Editing into Firefox Configuration files you can do this :-

• Open your profile folder:

             # Click the menu button , click on help and select Troubleshooting Information. The Troubleshooting Information tab will open.
             # Under the Application Basics section, click on Show Folder. A window with your profile files will open.

• Click the menu button and then click Exit .
• Open the file user.js with a text editor (like Notepad).

            # If there is no user.js file, these steps won't help you. 

Now Remove the lines for the options you want to be able to change. You can delete all the lines if you want to be able to change everything.

• Save the file and close the text editor.

You should now be able to change your Firefox settings. The Following thread helps you lot : http://mzl.la/Mt91fd

Reset Firefox {web link} is a LAST RESORT ! !

You may have a corrupt localstore.rdf file. https://support.mozilla.org/en-US/kb/changes-toolbars-and-window-sizes-are-not-saved Changes to toolbars and window sizes are not saved

Yeah, that didn't help at all. I ended up reverting to an older version of Firefox (30), problem solved.

In case one of your changes was to move "Bookmarks Toolbar Items" to a different bar than the regular Bookmarks Toolbar, that isn't working well in Firefox 31.

For general future reference: How to fix preferences that won't save.