I use it for sheer customization, and use Opera now because of sheer speed - maybe it is not noticeable to anyone else, but Using KeePass Password Safe as my trusted login manager, in IE the letters get typed at a rate of 2/s or 3 /2s, whereas in Firefox it is nearly instantaneous (well under a second for entire username and password, most of which are 34 or more characters) to be input.
Furthermore, loading pages is extremely noticeable between Fx 3 / Opera 9.5 and IE 7 - but a lot of that will depend upon your networks speeds as well.
The only fair, unbiased testing can come from multiple loadings of the same site in each browser, with all history, cache, etc cleared out - and then, in Vista, you also have IE in protected and non protected mode, which makes for a larger difference as well.
What it comes down to is this: As M$MVP Donna Buenaventura writes : There is no 100% secure browser. It is a matter of taste and personal choice. As soon as I can customize IE with all the same settings / tweaks / add-ons as I can with Fx, I'll give it serious consideration - Micro$oft has gone a long way to make IE 7 a lot more secure than IE 6 - but it is still only as secure as the user using it.
The one thing about Firefox (and a lot of FOSS) is that usually there is a much larger global span of developers looking at both vulnerabilities *and* patches - so that usually means fixes are quick to come out.
