All Mozillas are by default set not to release RAM unless it is needed.
Just minimize and maximize again - then You will see what the respective browser cannot release.
p. s. I opened two layers (this page &
http://kmeleon.sourceforge.net/download.php )
minimized and maximized again: result = 39MB Firefox and about 11MB K-Meleon.
Exact figures also depend on settings. The difference in RAM needed was AFAIremember bigger in the past. K-Meleon has the out of the box functionallity of a FFox with dozends of extensions. Next K-Meleon version will have true tabs - that is even more efficient.