I would say addons mainly. I can't live without my addons 
It is more down to personal preference I think.
If you just want to try FF, have a look at the portable version:
http://portableapps.com/apps/internet/firefox_portable
It installs to a folder, and allows you to try it without actually installing it. If you want to remove it, just delete the folder.
That's a good idea.
The portable version will be great so that you don't have to install it just to see if it's for you.
I will say that one of the advantages of it is the "higher security" that it provides over IE. It also has several built-in functions that aren't in IE or some other browsers. Such as Firefox sync. I use Chrome primarily, and it has sync as well, as well as internet printing (never really got that working though).
Add-ons are always a plus, but most browsers have them now. It's like comparing the Android Market and iTunes store (or whatever it's called) though. Firefox has a massive add-ons database as well as chrome, whereas IE doesn't.
