Ubuntu 10.10 on T41
I've been driven mad by this bug which happens when Ff is closed and then later an attempt is made to restart it.
The problem is apparently an extention issue, and is solved by closing the still running Ff in System Monitor every time.
I like Ff though, and I've just switched to a variant Firefox Aurora and it's not only faster but doesn't suffer from this issue.
More information including install instructions: http://ubuntuforums.org/showthread.php?t=1712247
EDIT - changed install link