Uninstalled all firefox AND deleted entire profile directory. Updated Thunderbird to the latest version. Reinstalled 4.0. Changed regular settings... no problems. Disabled all the plugins that were automatically added except for java, shockwave, and flash. Added download helper, tabmix plus, adblock plus, better privacy, beef taco, greasemonkey, and ghostery. Restarted with no problems. Configured plugins... then I had the same behavior. Killed the task, restarted, and then tried to close again. This time I get a prompt for "delete LSO cookies upon browser close?" I said YES and then closed the browser. Opened the browser, visited some web pages, and then closed with the upper right X and didn't have a problem! Solved? Not sure... but I think Better Privacy might have been the culprit... that if the configuration prompt doesn't complete properly, the plugin is trying to ask you a question before closing the browser but that question dialogue box never appears. Next step is to sync mozilla weave and see if I run into a problem. After that I will try changing the setting for security.ssl.allow_unrestricted_renego_everywhere__temporarily_available_preftrue which I need to get access to my citrix site at work (another bug, Mozilla?). Will let you know where I stand then.

Solved? Ok, so I think I fixed this myself. Originally, re-installing firefox after an uninstall (w/ option to delete profile) did not work. Here is what did work for me:
-Complete uninstall of firefox w/ option to delete profile
-Manually delete entire profile under c:/users/NAME/app data/roaming/mozilla (yes, information will still be there even after a complete uninstall of firefox)
-Update any other mozilla products (for me, Thunderbird... it turns out the older version was somehow sharing plugins and profile directories)
-Download firefox 4.0
-Install w/ defaults
-disable all plugins manually (go to add-ons and select "disable" for each... why are there plugins you ask? well, some things like Adobe reader, etc... will automatically load in when firefox is installed if that particular program is already on your computer)
-enable only basic plugins (for me, flash, java, and silverlight)
-change settings as preferred (my changes were all in the privacy settings)
-close and reopen firefox (I would recommend doing this at every minute step so you can troubleshoot what happened)
-add plugins/extensions one at a time, reset firefox (option given once a plugin is installed), configure the plugin, close firefox, open firefox, repeat and wash for each plugin...
-if any plugin/extension causes a hang, disable it. "End tree" in task manager for the hung firefox. Start firefox. Disable plugin. Do not "Restart" firefox at this point, as it will hang. Instead, use firefox-->close. Then, go in and try enabling the plugin, configure it, and the restart. If all is good at this point, leave the plugin. If not, wash, repeat, leave it disabled and move on without it at this point.
-run your firefox sync
-enjoy a working copy of firefox.

The essential point is to CLEAN EVERY POSSIBLE THING OUT and install the bare minimum, then add parts incrementally and determine that every increment works. Simple, straightforward, but not always obvious.

I hope this helps...

Problem is back.... without installing new extensions, the problem seems to be coming back on occasion. It seems that when firefox is left on for a while, and many sites are visited, it needs this "Wash and rinse" of a manual restart and a forced shutdown through the task manager. I have no idea what the problem is at this point. Why my previous solution worked for a while is beyond me. Perhaps something is being cached by an extension. I do have my browser set to clear everything at exit.

Any suggestions?

Solved !!!: Downgrade to Firefox 3.6. FF4 is just unstable. When I came across the same problem, across different machines, including my laptop, all running Win7 x64, I decided that aside from putting my tabs at the top, FF4 hasn't done anything to my browsing experience except ruin it from instability. With 3.6 and all of my regular plugins, I get all the security I need, without the hassle of FF4 instability. Furthermore, until there is actually new content out there that takes advantage of the newer browser features, most of FF4's sparkle has no light from which to shine.

I have the same problem with Firefox 3.6 and have been unable to find a solution.

FF5 solved the problem. I guess 4 and 4.X were still not really out of beta.