From your System Details, it looks like user.js is currently active in Firefox. The user.js file is only present if you or other software has created this file and normally it wouldn't be there.

You can delete a possible user.js file and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release. You can get to the profile folder below:

• Help > Troubleshooting Information > Profile Directory > Show Folder

or

• Go to about:support and click Show Folder next to Profile Directory.

If the above doesn't work, start Firefox in Safe Mode to check if one of the extensions (orange Firefox button, top left corner (The image "new fx menu" does not exist. for v29+)/Tools > Add-ons > Extensions) is causing the problem (switch to the DEFAULT theme: orange Firefox button, top left corner (The image "new fx menu" does not exist. for v29+)/Tools > Add-ons > Appearance).

• Do not click the Reset button on the Safe mode start window or otherwise make changes.

• Windows/Mac: Go to Help > Restart with add-ons disabled
• Linux: Run firefox -safe-mode in the Terminal/Konsole

Note that you currently appear to be running Firefox in Private Browsing mode

• https://support.mozilla.org/kb/Private+Browsing

• Tools > Options > Privacy > Firefox will: "Use custom settings for history"
• Deselect: [ ] "Always use private browsing mode"