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")

safe mode was no good. still had the problem. i'm worried that this might be something in the windows shell, but i'd made no system changes or recent installs when the problem began. (at least this time... i can't recall what may have preceeded tne previous two).

i'm going to give it a shot in windows safe mode and see what happens.

windows safe mode didn't help either. argh.

I have the same problem since about 6 months. I have no problem with Thunderbird. I tried all the solutions I found on the web. I switched to Firefox 4 beta 9 (now 10) and miracle! it worked again.... but only for 3 days... :-( so the problem is the same wathever the version of FF.

found out what was doing it!

after trying firefox safe mode and windows safe mode and any number of other things i'd kinda given up for a bit there, but i woke from a dream in the middle of the night thinking "based on the behaviour involved it *must* be something in the shell that's causing this." (sorry, i dream in geek.)

so i grabbed a shell viewer editor - ShelleExView (http://www.nirsoft.net/utils/shexview.html) (had to use an icky, lesser browser to be able to download it) and gave it a run.

i listed all the extensions by the date they were added as a clsid and tried to think back to when the problem began... about a month or so after a fresh system reinstall. and through a little trial and error disabling i found the culprit.

it was an icon overlay handler put in by something called WagerLogic. after looking into the company name i'm going to have to assume this was related to one of the annoying "come lose real money at our virtual casino" pop-ups that infest the net nefariously panning for an easy mark like dickensian orphans.

i disabled it et voila! firefox is back to full function again.

I just recently started having this problem. I am running Firefox 4.0.1 on RHEL 5.6.0. I can open the Donwloads window, and I have cleared out all of the entries. But whenever I do anything that should open the Save As dialog the entire application hangs.

Firefox is not to blame in my circumstance. It turns out that my problem was caused by the io subsystem of my workstation getting swamped by another user's processes. This caused the directory listing request of the firefox file chooser to hang, which is why the entire application froze.

In my case Firefox hangs when trying to open tools/options or downloading a file. A program called TortoiseCVS cause this problem. i've used ShellExView utility to disable it. Now firefox works fine