The Firefox IFRAME-popout bug also happens on Hardy, and has been an issue since Firefox 1.0 and earlier - see http://lwn.net/Articles/357074/ for link to the Mozilla bug.
Interestingly, I don't get this bug any more since switching Hardy from a normal 32-bit desktop 2.6.24 to a PAE kernel, gaining access to about 800MB RAM as a result - perhaps the reduced memory pressure or something has made a race condition less likely. However, I do get frequent crashes which might be another manifestation of the same bug.