Mozilla engineers have began and effort known as “MemShrink” to reduce Firefox’s rather large memory footprint, with the changes being made in Firefox 7. (I still haven’t quite figured out Mozilla’s new version numbering scheme, and Firefox 5 is the current release…)
Firefox 7 uses less memory than Firefox 6 (and 5 and 4): often 20% to 30% less, and sometimes as much as 50% less. In particular, Firefox 7′s memory usage will stay steady if you leave it running overnight, and it will free up more memory when you close many tabs.
Also, there are already some early mock-ups of what the UI in Firefox 8 might look like.
Firefox 7 is lean and fast [Nicholas Nethercote]