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).

• Don't make any changes on the Safe mode start window.

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.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

I had this problem too and starting Firefox in the safe mode (as well as disabling all add-ons) did not helped. It's strange, but I've never noticed this bug in the 3.6.13 nightly builds, which I will use for now.

This bug has been here for ages and never gets fixed just like a ton of other bugs, including annoying hangs and crashes all the time. I like Firefox but there are too many bugs that basically never gets fixed, been there for YEARS. For this I have quit using Firefox switched to Chrome.

Thanks for posting a reply here, which reminded me about this question. I figured, there is a problem with the KDE file picker dialogue (at least). If you still experiencing this bug, go to about:config and toggle "ui.allow_platform_file_picker" parameter. It must be set to "false". This will get you not-so-good-looking internal file dialogue, but it works.