Rated 1 out of 5 stars
@NEmom, It's a plugin, so it would be listed under plugins, not extensions. If it's not there, then beats me (I see it in my plugins). The installation may not have worked properly.
If you can determine that it has been installed properly, then go to Firefox-->Preferences-->Applications and for the "Preview Document (application/pdf)" content type select "Use PDF Browser Plugin 2.4.1 (in Firefox)" or "Always ask" as the action.
That's weird though, it's been working for me even without me doing all that. So I think your best bet is that the installation went wrong somewhere. *shrug*