This isn't very good advice but it might help ya out
Go to your bookmarks organizer and export your bookmarks, you'll want to save them so you can import them later.
If you don't have a lot of firefox customizations you can open a terminal and mv -i ~/.mozilla/firefox/ ~/my-mozilla-backup
You can move it back if ya need to, remember backups are a good thing!
This'll move your firefox directory out of the way, so firefox will make a new one the next time you start it, minus any junk Adobe Reader 9 could have stuck in there.
If you do this you'll lose your customizations but it's the easiest way to get rid of something that's embedded itself in your firefox that you can't easily get rid of.
You'll also wanna check your mozilla plugins directory usually it'll be in /usr/lib64/mozilla/plugins or /usr/lib/mozilla/plugins or somewhere under /usr/ -- see if adobe reader has a plugin there.
libnullplugin.so you'll need to keep, the rest you can blast, unless it's something you need. If you've previously installed flashplayer you'll see libflashplayer.so in the plugins directory. Don't delete it if you want flash.
This isn't necessarily the best way to do it but it's probably the simplest.
If reader is embedded somewhere else this won't work.
Kids, please don't try this at home.
Good luck