I absolutely can't stand non-tabbed browers now that I use tabs. It makes so much sense that its amazing nobody thought of it before. It is honestly a make or break feature. Middle mouse button clicking to open in a new tab. So convenient. Built in pop-up blocking is wonderful, often beats the ad blocking of the Google Toolbar for IE. The Mycroft search feature is amazing. Instant access to Google, Ebay, Amazon, Packetnews, Dictionary.com, Wikipedia, etc. The skinning and extension systems are wonderful. My personal favorite is the Web Developer extension, which saves me loads of time. Firefox renders insanely quickly as well. I have personally never experience a crash with a release build of it.
As for Internet Explorer, it is missing all of the aforementioned features. It is also slow, buggy, and insecure. There are far more exploits for IE than Firefox, and Microsoft is slow to patch it. It is true that a few sites are IE only, but shame on them. Firefox works better on many pages, especially those using .png files. IE just butchers them.