I've too been noticing extreme slow running with FF.
Nothing shows up in performance monitor so it looks like the 99.x% CPU time is being spent somewhere at low level in the O/S (XP, SP3).
It may well be related to certain websites which use weird javascript or even java. I had this issue with flyingexam.com which uses massive amounts of javascript to obfuscate the questions so they can't be grabbed easily, but Vimeo may be doing it too.
It is very bad that in 2011 this still happens. You would think windoze would time-slice in some sensible way e.g. not allow any one process have 100% of CPU time especially as it is not shown in perf monitor which one is doing it (but, obviously, closing FF, eventually, does fix the issue).