This problem actually happened to me when I used an old version of both Firefox and Ubuntu over a year ago, but definitely not in Lucid and Maverick. It's now back on Oneiric (currently using the beta Firefox 9) on a Sony sz450, and in Natty using the stable version on a ThinkPad T60. I really wish I could find a way to reproduce it. I think it might have something to do with workspace switching, but I cannot see a pattern.
This problem actually happened to me when I used an old version of both Firefox and Ubuntu over a year ago, but definitely not in Lucid and Maverick. It's now back on Oneiric (currently using the beta Firefox 9) on a Sony sz450, and in Natty using the stable version on a ThinkPad T60. I really wish I could find a way to reproduce it. I think it might have something to do with workspace switching, but I cannot see a pattern.