Do you even know what you’re talking about?
OOB IE7 in Vista is double-jailed [also read as running in a “sandbox”]… UAC and Protected Mode. You can disable both of these security features however if you can live without disabling them you have the most secure OOB browser.
Any browser you install and then customize will (usually, granted your not modifying the browser to increase security) increase the chance that malicious code will execute and infect your computer.
Using FF and then tossing a few extensions on is not safe. One cannot assume FF extensions are regulated for security let alone programmed well…
If ActiveX is your argument then I suggest you don’t use FF or Chrome… Both can execute ActiveX. [Google Chrome installs and ActiveX shim (by default) and FF uses XPCOM to execute ActiveX code… ]
Listen, you can install FF or IE7/8, Chrome, etc and still get a virus. If installing FF on XP helps you sleep better… ignorance is bliss, I guess.