Installed FF3 the other day, all i can say is CRAP.
Literally takes more than 4x more ram and 10x longer to load. Some pages just pause for like 15 seconds, THEN decide to slowly download. And visually?... diabolical.
A bloated mess IMO.
Then there is something wrong with your install, or just your system in general.
I've used firefox and IE on several machines, ranging from an 800MHz Athlon to a Core2Duo, with various 32 bit and 64 bit Operating Systems.
And there isn't a single case where I've seen either IE or Firefox use anywhere near 4x more RAM than the other, or be 10x slower than the other. And I've certainly never seen either of them 'pause' unexpectedly, apart from on my oldest system, which isn't surprising, as most applications suffer from cpu load on such an outdated system.
From the various combinations I've tested, just in terms of speed and and response, I'd rate them in the following order:
FF3 > IE7 > FF2 > IE6...
But that being said, the differences are marginal. If you're seeing anything worse than about a second to startup, or to load a page, then you've either found a very extreme case (with a specific bug) or you've just got something else wrong with your install imo.