A thousand times no! I barely trust the distro people to not screw up the browser itself. The only reason to use a distro-packaged extension would be that it's more likely to be fucked in lockstep with the browser.
I'm sure you use CPAN, gems, hackage, etc. instead of the broken crap in nearly every distro's repositories -- why not use http://addons.mozilla.org while you're at it?
I use OS packages for everything. Fixing the occasional breakage there is much easier than the constant manual updating and tweaking required to maintain your own installation of everything.
I have a few local Emacs Lisp extensions, and I maintain my own ~/perl5 and ~/.cabal... but I bootstrap them with the OS packages; various emacs extensions, the OS perl and liblocal-lib-perl, and haskell-platform. Modules my own software needs goes into ~; modules that OS-managed software needs I leave untouched.
Basically, if your distribution breaks your packages regularly, you should get a new distribution.
I'm sure you use CPAN, gems, hackage, etc. instead of the broken crap in nearly every distro's repositories -- why not use http://addons.mozilla.org while you're at it?