Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

You can use "Disable all add-ons" on the Safe mode start window to disable all extensions.

You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

I had this problem too and found out, it was related to the custom search engines that I had added with the add-on "Add to Search Bar". Somehow they got corrupted.

Make a backup (a copy) of the folder "searchplugins", inside your Firefox profile folder, then in the search bar click on "Manage search engines", then on "Restore defaults".

Close Firefox, empty the folder "searchplugins", open Firefox, close it again, put all search engines files from the backup folder back into the original "searchplugins" folder inside your Firefox profile. Open Firefox.

That solved my problem.