This is a weird bug. I'm surprised it persists.
Firefox 3 won't run Add-ons that aren't compatible with it.
That's fine, but it won't let you uninstall or remove them either. I think FF 2 did, so this is a new bug.
It's a cosmetic annoyance since the incompatible add-on isn't doing anything, but I'm surprised it hasn't been fixed. It's not exactly subtle.