I am running FireFox 3.5.6 & the latest version of CCleaner, with Vista which I use as my default cleaner. Some time go I also bought the SBMAV Disc Cleaner. Before closing my PC for the day I ran as I always do CCleaner, I've felt for some time it cleans FF rather too quickly after a long browsing session, & tonight it took seconds.
So I ran SBMAV in full scan not the usual quick scan. This brought up around 50 meg of FF cache, some many days old.
This isn't really a problem for me & no doubt will be sorted in a later CClean build. But if you are relying on CCleaner to remove sensitive information from FF it may not be protecting you as well as you think. I have also just duplicated the problem with my old DELL Latitude running XP.
This isn't a criticism of CCleaner, & it may not be an issue with your PC, but it might be worth looking at your FF profile after running CC.