Delete sessionstore.js and sessionstore.bak in the Profile Folder

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

You will have to redo App Tabs and Tab Groups after deleting sessionstore.js.

See:

• http://kb.mozillazine.org/Session_Restore

Set the pref browser.sessionstore.max_resumed_crashes to 0 on the about:config page to get the about:sessionrestore page immediately with the first restart after a crash has occurred or the Task Manager was used to close Firefox.

That will allow you to deselect the tab(s) that you do not want to reopen, but will allow to reopen other tabs.
See:

• http://kb.mozillazine.org/Session_Restore#Restoring_a_session_after_a_crash

This didn't fix the problem, however I am able to recreate the problem and repair it.

To recreate the problem, the only thing that needs to be done is start a download and Firefox starts locking up. After a while the Firefox window disappears and a new Firefox window is opened with only one tab that points to mochimedia.com. Then after a while that window disappears and the old one reappears with all the old tabs in it. Sort of like a hidden window that only pops up when Firefox is freezing up. I know 4.0 is a new version, maybe it is a bug that needs to be worked out. I am thinking because the download folder has some json feeds in it from Mochi maybe Firefox is trying to re-establish the connection to finish the download it crashed on.

Since I had a ton of junk in the download folder all I did was delete some and move the rest. This seemed to remedy the problem.