Recently I upgraded Firefox on our Windows XP machine to version 3.6. Today I noticed that there were lots of versions of Java Console in the list of add-ons and it occurred to me that these might be slowing Firefox down (maybe Firefox was attempting to load them all on start-up?). Anyhow I haven't used Java Console for a very long time (it is basically a tool software developers) so I went to uninstall them, but their Uninstall buttons were all greyed out. Then I remembered that I installed Java from an administrator account, so I logged in as Windows administrator, started Firefox, and sure enough, I was able delete all the versions of Java Console from there.
Incidentally, you might also find Java Quick Start in the list of Firefox add-ons. The sole function of this add-on seems to be to make Java start-up faster by making sure the core Java files are always in cache. Of course this has the side-effect of slowing other things on your computer. The best way to disable Java Quick Start is via the Java control panel in the Windows start menu:
Start > Settings > Control Panel > Java > Advanced > Miscellaneous
and then untick 'Java Quick Start' (you can always re-enable it if you really find that Java is unbearably slow in starting up). You might also like to untick 'Place Java Icon in System Tray' while you're at it.