Basically, the advantage of FF is the customization. If you are fine dealing with IE's normal interface, enjoy. I have customized FF a lot so far. In the Status Bar, I have Adblock, which blocks specfiic ads i tell it to. I have Homeland Security Threat Level, for kicks. I have Foxy Tunes, where i can control ANY music player. Name it, it is probably able to be controled. And i have Weather Fox. I see what temp it is now, and tonight, but you can add a 5 day forecast if you want.
I have a couple of other Extensions, such as mouse gestures, which i am just getting used to, Download manager tweak, which adds it to a tab, Translate, which you can click a button and it tranlsates any page, and Image Zoom, which lets me click on any image, shrink it, enlarge it, etc.
To sum it up, It is worth trying out. I am still getting used to it, and its been a couple months. You can not try it for a day, say "its the same" and then leave it. Thats wrong. I am just now getting into features i didnt know i could use. Ive added search engines into the upper right field, such as IMDB, Ebay, Wikipedia, Dictionary.com etc. I am still customizing mouse gestures to make my browsing easier.
It may or may not be more secure. If FF has half of the market, and IE has half of the market, only then will we know which is 'more secure.' It may be faster on your system, it may be slower. For mine it is faster loading webpages, and sometimes faster loading up. It does have trouble with *some* webpages, but that is why i have a plug in called IE view, which you right click on a link and it says Open in IE. And it will load IE up to view it. Its all up to you. What do you want Your browser to do?
Edit:
Oh yea, I recently redid one of my roomates computers. I took off all links of IE off, and installed FF. no extensions. I made it the default browser. He loves it. Havn't heard a complaint yet. He says it seems faster then before, but of course, that is right after a reinstall of a computer. Point is, you can switch back and forth between IE on one computer, and FF on another, and barely notice.