It's a common install issue with Windows 7, requiring removing all but 2gb of RAM to complete installation.
In your case I would thoroughly test the RAM following Jonathan's steps given here to test both RAM and slots: RAM - Test with Memtest86+
Firefox problems could be related but also are often solved by reinstalling or upgrading Flashplayer upon which it is heavily dependent.
I would then work through these troubleshooting steps
to look for repeat errors in logs, fully utilize System Resources, complete hardware testing until you winnow out the problems.