Sorry, but it is a leak.
Three completely different Firefox profiles, one on default settings, the others tuned and tweaked for various purposes, to the point that I doubt my normal browsing profile has any significant parameters in common with the default setup. Memory usage in all of them grows with each page loaded or reloaded, and never goes down, even after the tab or window into which each page was loaded is closed.
There's no reason for the memory usage to gradually grow by four times the disk cache size or more when the set of pages being displayed is unchanged. "Leak" is the only conclusion that makes sense. It's possible that it only affects the Linux, or even the 64-bit Linux, version, however.