I was able to get the Firefox PDF Plugin to work with firefox 4 by following the instructions at
It has the unfortunate side effect of forcing you to run FF4 in 32-bit mode, which may be a problem for some other extensions (for instance, I believe 1Password now requires 64-bit support).
Basically, there are several problems which broke PDF support on FF4:
- Firefox PDF Plugin relies on some 32-bit libraries. Since FF4 runs in 64-bit mode be default, this prevents a straightforward update to the plugin. The instructions above contain a beta version of the PDF Plugin, but I'm sure work is progressing on an official 64-bit solution.
- The Adobe Reader X plugin (for mac) only works with Safari, so this can't be used with FF4.