I use Firefox simply because I have used it since forever. I remember when I was young and figured I should not be using the Internet Explorer: I tested out Opera and Firefox and went for Firefox. Since that time I've been a loyal fan. I don't even think Chrome was a thing at that time.
Ironically I tend to find a lot of useful add-ons that are only available for Chrome, so I've been tempted to change browsers to be honest. However, I've never gotten around to it, and at this point I feel like I would be backstabbing Firefox :P