I get this about 50% of the time, and I have noticed something that correlates with it 100% of the time.
That is that - for some unknown reason - the entire current page view will paste itself into the quick reply box at the bottom of the thread (whether or not I had any intention of posting a reply). It's like a screen shot of the whole forum page I happen to be on, suddenly appearing in the quick reply box. If I then want to close the page, or move to another page, I get that "Do you want to ....?" question.
Similarly, sometimes when I actually initiate a reply in the middle of a thread (say to a post that is not the "lowest" post in a thread), the whole page will sometimes similarly paste itself into the reply box. Of course I can't type a reply there then, so I usually close/re-open the "reply" four or five times until it loads with a blank box ready for me to type in. Or sometimes I have to reload the whole thread page and try again.
I was thinking about taking a screen shot and posting about it, but then I didn't see anyone else mentioning it, and so I decided to live with it and not bother anyone (until I saw this thread). I can take a screen shot next time it happens and post here, if that would help.
I'm on OS 10.6.4 and Firefox 3.6, btw.
herman: Have you noticed if you also get the "full page screen shot" in the reply box when you get the "Do you want to ....?" message?