For some of us .pdf files have been troublesome within Firefox - causing freezing and crashing.
If this is you - the good news is there's now a fix.
This first link is the Mozillazine thread: Mozillazine
This second link goes to Adobe Reader download page: Adobe Reader
I went the route of electing to replace Acrobat 6.0 with the new version 7.0 (free/basic version) and have found it has cured my freezing and crashing problems.