There’s been some confirmation from users that Firefox 5 has fewer leaks than Firefox 4.
From bug 640923:
Well, FF5b2 is really doing way better in my environment. This is great! I’ll abandon FF4 now for me, it never was “stable” in my world.
From bug 657232:
I’ve just upgraded from Firefox 3.6.17 to Beta 5 and it at first appeared to be a lot more stable and responsive than Firefox 4.
Nice to hear!
“Fewer memory leaks” (or “reduced memory usage” if we want to be less blunt) should definitely be on the Firefox 5 feature list when it comes out.