I’m using XP, and when developing, like all of you, I’m switching back and forth between FF (for dev tools) and IE (to see how the page will render for the majority of users).
Now, FF doesn’t have any native font smoothing. IE does. So, text will render differently in the two browsers. Especially italics, which can look horrible in FF.
You can make FF display smoothed text by turning it on globally in Windows’ display properties > effects > smoothing > cleartype. Now IE and FF look the same.