On my 512MB Raspberry Pi, this used to be FF's greatest strenth. Unfortunately the latest versions are getting nearly as bad as chrome. I wish Mozilla would focus on their strengths and put more resources back into their memshrink project.
When Chrome started to get fat, it was great to have a lightweight alternative like Firefox. But lately even Firefox has been causing my 1GB netbook to thrash. I hate to have to junk a perfectly good machine that is only used for email and browsing just because of software bloat. After doing a factory restore, it looks like that or a lightweight Linux distro(while losing flash and HW accel) are my only options.
There are many memory usage improvements made in firefox that don't appear explicitly in the areweslimyet tests, and there are many others, like better memory usage reporting accuracy and new features that do, thus the apparent increase in memory usage throughout the latest versions that you pointed.