Hi,

> as plausable causes.
> 
> I'd still like to know if it affects 3.6.4 on any other machines!
> 

I have exactly the same problem, so it isn't just you.

For me the problem addon is Xmarks. If I have this installed, firefox 
3.6.4 on SLC5 does not start. If I delete the extension by hand (how you 
describe) it starts just fine. If I reinstall the extension I am oddly 
allowed one restart where it works, but then I am back to firefox 
starting on subsequent restarts. Other extensions (like adBlock) are OK.

If you search the web, there seems to be reports of problems with 
firefox 3.6 that sound very similar to this, e.g.

<http://support.mozilla.com/en-US/forum/1/590191>

That is for OSX, but otherwise is very similar to what I am seeing.

Chris