15% faster is it? I can't say I have noticed but it certainly isn't any slower.
The main thing I noticed immediately was that I had an add-on called YesScript to cure the blue margins on the PCA site and it wouldn't work with the updated version but it didn't matter because the blue margins have gone now.
I also had an add-on called greasemonkey (I think it had something to do with the 'intelligent text' type adverts on sites like PCA) and that wouldn't work with the updated version but it doesn't matter because I don't get those problems either any more.
I am very new to Firefox and have only updated a few times. It seems that each time there is an update a lot of the add-ons lag behind, which is a bit different from IE, which is developed as a package before release.
On this occasion the add-ons I was using are no longer necessary and Firefox is significantly faster than IE.
Before changing to Firefox I tried out Google Chrome, Safari and Opera and satisfied myself that, overall, Firefox was best.
I find no difference between IE7 and Firefox, I have very good download speeds (6.5MB Average) so I think that will make a big difference. Howevr I still prefer IE. All a question of personal preference if there is no percieved difference.