I work on a few different computers and have dabbled with using Firefox as my browser on some of them. I have now discovered the killer feature that has made me a total convert.
No, it's not tabbed browsing, although that is really quite good. (If only it were 100% implemented instead of 90%!) Nor is it popup blocking, which is also very good, but just as well implemented by the Google toolbar. Nor is it all the other things the Mozilla folks justifiably blow their own horn about.
For me the killer feature is that when I press Ctrl-+ the text on the Web page gets bigger. When I press Ctrl-- it gets smaller again. What is great about this is the text changes size in a useful way, not like the lame text size menu item in IE. The keyboard shortcuts are so fast and convenient (and mnemonic) that I find I use this feature all the time.
This feature is just great for my own middle-aged eyes. I predict it will cause a huge switchover of baby boomers to Firefox. I'm going to install it on my mother's computer too, next time I visit.