Very late reply, but I tried win7 x64 and x86 with 4GB. Taskmgr shows the ram usage jump from 900MB to 2.5GB after a fresh reboot.

I tried downloading the entire page (html,css,js) and increasing TOTAL_MEMORY (et al) in the js, but with the same result.

Nightly-23 from 07 May (firefox-20.1, same result). It did work on ubuntu-amd64 though.