I assume your using v2.0 and apparently there is a few bugs regarding this, from Mozilla's forums clicky.
I'd suggest as the last poster replied disable each of your plugin's in turn (also make sure they are all updated).
You might want to concider downgrading to version 1.5.0.8 until they get it ironed out (it still in 'kinda' beta) if you really need the offending plugin (if of course this turns out to be the problem)
Thanks Minkey.
Yes , I'm using v2.0 and it does not happen all the time ......I've only had my PC recently reformatted.......and installed v2 immediately and had that message appear about 3 times. I noticed that Firefox found an update to my skin that I installed and I reckon that is why I got the message at shutdown. Anyhow , I have only had the message 3 time and might just leave things the way they are. I have disabled any future updates as this is where I think the problem is.