It is probably some plugin that worked perfectly under 3.0 that is not playing well under 3.5. Try disabling all of your plugins and re-enabling them one by one. For me it was BetterGmail that was crashing FireFox. Disabling it fixed everything.
Firefox is pretty useless to me without firebug -- extensions can't be disabled/enabled without restarting, and it seems impossible for any nontrivial extension to noop itself because of the event handling nonsense. I really don't want to have to always be using multiple profiles, but it might come to that.
I have no reason to doubt it is true; I know poeple who claim to have made Firefox run much quicker by disabling the cache or making the cache direstory read only.
One of those people uses a 56k dialup connection that rarely connects as fast as 56k.
How badly do you have to screw up a cache implementation to get it where it is faster to fetch the data over a 56k dialup than look it up on the local harddisk ?
...
...
... a very long time to start up. :(
I did have a few weeks worth of horrible Firefox instability last month, but it turned out to be due to faulty memory in my desktop machine.
Now I've switched to Chrome but I do miss the Flashblock plugin.