Whenever a new version of Firefox rolls out, as 3.5.3 did last week, there's always a few glitches, especially when it comes to extension handling. Before rolling back to an earlier version or whining on the Mozilla forums, it's worth checking the settings for any offending extensions.
I was reminded of this when the bug489729 extension, which I find essential for disabling tab tearing, appeared to have stopped working after 3.5.3 installed. Turns out that installing somehow turned off the settings I wanted, but didn't actually break the extension. Worth looking into if you've had extension woes, especially if you've also disabled extension checking to keep older favourites working.