Why does FF open two windows when I open it? Why can't I close the second window without closing FF altogether?
After FF automatically updated it started opening two windows whenever I start FF. One window has the open tabs from my previous session and the other window has only the FF home page open in one tab, nothing else. I can't close either window without shutting FF down altogether. I have limited RAM (3GB), running Mac Lion, which eats up a lot of resources and now FF eats up the rest. It's a real pain in the ass.
Try with Firefox in safe mode. If it doesn't work, create a new profile and transfer your data to the new profile. See http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox
It is possible that there is a problem with the files sessionstore.js and sessionstore.bak in the Firefox Profile Folder.
Delete the sessionstore.js  file and possible sessionstore-##.js  files with a number and sessionstore.bak in the Firefox Profile Folder.
- Help > Troubleshooting Information > Profile Directory: Open Containing Folder
Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost, so you will have to create them again (make a note or bookmark them).