Firefox 1.5 uses memory in proportion to what's available on your system.
Depending on how much ram you have, it will store a certain number of pages
in memory to achieve the faster forward/back caching. On my Win2k with
512mb ram, Firefox will use between 50-60mb ram consistently. I have not
seen it rise above 60 ever, even after hours or days of use. On systems
with less ram, I suspect it will use a little less. But in general, you
can expect it to use a fair share of ram yes. With IE, I believe parts are
already loaded with the OS, so you won't see such high usage in Task
Manager, or at least that's the story everyone tells. There are some
about:config tweaks to adjust ram usage I believe, as there are ways to
adjust it in Opera too. But in general, you can probably expect the modern
browsers to use more ram than IE.