One reason that browsers are using more memory over time is that browers are way more capable than they used to be. HTML5, web apps and all that.
People say "I have N tabs open, Firefox is using M MB of memory". Often followed by "that seems outrageous" or "that seems reasonable". Without saying what those tabs have in them, it's pretty much useless.
Firefox's memory usage has varied over the years. FF2 was terrible, and contributed heavily to the bad reputation. FF3, FF3.5 and FF3.6 were much better. FF4 is worse than FF3.6; FF5 will be better (it already is). Subsequent Firefoxes will hopefully be better again.
Is this per-tab or global?
It's global memory usage at the moment. I'm working on https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fbugzilla.mozilla.org%2Fshow_bug.cgi%3Fid%3D661474 which will give JS memory usage per "compartment", which roughly means "per domain". So if a page at foo.com has a Twitter feed and some Google Analytics stuff, there'll be three compartments, one for foo.com, one for twitter.com, and one for google-analytics.com. It's not quite per-tab, but in that direction. about:memory is evolving, keep an eye on it.
The first sign of maturity is the discovery that the volume knob also turns to the left.