This is what I got EVERYTIME I close Firefox now on my Macbook:
Click to view attachment
Firefox crash upon I exit it.
This goes with few slow down when I use it and few lock downs.
And worst of this, few features stop working, IE: I can't right-click and select bookmark or view source as it don't work.
Now you will tell me that I should just re-install it, and there goes the problem.

I brought my MacBook half November and since then, I re-installed Firefox (or a derivative) many times for more or less the same troubles as I have right now.
Starting with a regular FX 3.0, went to 3.1Beta2, then Flock 2 and the current Fx 3.0.
This is 4 FX installation in 6 months! In all cases I started from scratch importing only my Bookmarks.

Why Firefox is so laggy/unstable on OSX?

Just to tell, I had keep the same Firefox install for more than a year in Windows even when reinstalling Windows (More than 6 times), I was just saving the different Firefox folders and settings to use in the new Windows install.

This really upset me, FX is the only browser that suit my needs with the extensions, UI and I don't want to change it.

Go through this diagnostic: http://kb.mozillazine.org/Standard_diagnostic_-_Firefox

Thanks for the link! Good stuff to read.
Unfortunately I don't have any of the "bad" extensions.
But I will look more into extensions, this week when I will do a clean reinstall (for the 4th time), I will give up on many extensions maybe one of them work bad in OSX and not in Windows.