You blame Firefox 4 to be a memory hog? Check it out first by typing about:memory in the address bar. You'll get a nice detailed report of your browsers memory usage. While it's not guaranteed you'll understand every statistic available in the report, you can at least peek at the overall memory use, and see how much it's fragmented by comparing "memory mapped" and "memory in use" numbers.

I definitely see memory usage of my browser only go up during regular use, but it's not really problematic, considering there are gobs of RAM in today's computers. And there has also been a steady improvement in memory usage efficiency, 3.0 has been the biggest hog for me, 3.5 was quite an improvement, and now 4.0 is absolutely the best of them all. Alive and kicking (though, much of the responsiveness improvement comes from better javascript engine, I suspect). It's good to know that Mozilla plans even more improvements on that front with Firefox 5, 6, 7... which they advertise will be released this year!