I've seen and heard a lot of reports from many different Firefox users about Firefox using an unreasonably large amount of memory, even when using fresh installations of the most recent version, and when engaging in very reasonable browsing patterns.
As a software developer faced with a large and frequent volume of reports of such a nature, the only responsible thing to do is to assume that there is truly a problem. This should be assumed even if the developers themselves may be having trouble reproducing the problem. Denying that the problem exists is usually the most counterproductive thing that can be done, because the problem likely does actually exist, and it doesn't get fixed.
By the way, I don't believe that there ever was a Firefox 2.5 release. Perhaps you mean Firefox 3.5?