Although I agree that browsers in general are "consuming more and more memory", I'm seeing a slightly different story than you are. I abuse Firefox 4's new tab grouping functionality horribly, and keep tons of tabs open regularly. I use to have to pare them down just so I had room to read the titles on my tab bar, but now I can have upwards of 50 tabs open at a time.
Right now, I've only got 15, including my 5 pinned "App Tabs" (Four Google apps + FB). My Firefox is consuming 475 MB.
I was curious, and decided to see how much RAM Internet Explorer 9 would take with the exact same 15 tabs open. It appears to have spawned four child processes totalling approximately 1,283 MB.
My data: http://i51.tinypic.com/fualq0.jpg
(Sorry, no pic of the tabs I've got going, you'll just have to believe that I really did open all the same tabs)
And yes, I should include data for Opera and Chrome, but I didn't have those installed, and want to get this posted in time for a chance at being modded up. ^_^
Personally, for how extensible, and how many great features Firefox brings to my online life, I've never considered its memory consumption unreasonable. And it certainly appears to be doing better than the competition.