My computer can handle even older builds of Firefox perfectly fine (it only gets sluggish after I've left my computer on for ages and I have over one hundred tabs open), and modern builds have almost eliminated the significant performance issues that have tarnished its reputation. I made the switch back when Internet Explorer had almost complete market dominance, IE hadn't been updated in a long time, and Firefox and Opera were the only prominent alternatives. IE didn't even have tabs then, and I'd typically have 40 or more Internet Explorer windows stacked on my taskbar -- yes, switching between them was hellish. Firefox blew my mind, as did Opera (I used Opera for quite some time as well). Fortunately, Firefox and then Chrome gaining popularity have forced the browser market to be a lot more competitive, but I still find Firefox the perfect solution for me -- after all, people's needs differ, and Firefox allows you to cater to all of your needs in the way you choose -- even after having investigated most prominent alternatives.
I prize extensibility in a browser. I use many addons as an integral part of how I use the internet (because most applications I use on the computer are internet-based). I really feel Firefox is second to none in customisability and, having spent significant time configuring everything to my exact specifications, I won't switch. However, I do have other browsers (such as Internet Explorer, Chromium + Chrome, and Opera) installed on my computer for compatibility purposes.
I use Safari on my phone though ;p Although I'll be making the switch when I get a new phone.