Firefox developer Nicholas Nethercote outlines some of the changes. The first is per-compartment reporters, which should help power users and ordinary users see what's going on:
One nice thing about this feature is that it gives technically-oriented users a way to tell which web sites are causing high memory usage. This may help with perception, too; people might think "geez, Facebook is using a lot of memory" instead of "geez, Firefox is using a lot of memory".
... in short, the size of the heap was over 5x smaller (21MB vs. 108MB) after closing a number of tabs and forcing garbage collection. Even if you don't force garbage collection, it still helps greatly, because garbage collection happens periodically anyway, and longer browsing sessions will benefit more than shorter sessions.
This change will help everyday browsing a lot. It will also help with the perception of Firefox's memory usage - once you learn about about:memory, an obvious thing to try is to browse for a while, close all tabs, and see what the memory usage looks like.
I come across a lot of people complaining about how Firefox uses memory, so this will come as welcome news to them.
What's interesting here is the two-fold approach that Mozilla is taking. It's been a long-standing complaint leveled against Firefox that the browser uses too much memory and that it doesn't release memory efficiently when tabs are closed. Here Mozilla are not only working hard to shrink the amount of memory Firefox uses, but also giving users the tools to see how much memory the browser is using, along with the all-important 'why' it is using that memory.
What else should Mozilla be working on for Firefox 7?