I’ve been using Firefox since about 0.93, with about a year in the middle when I used Chrome as my primary browser, because it ran from a USB disk much better than Firefox did at the time, and that was what I needed at the time. (I am glad that Chrome came along, because by showing how bad Firefox was in some areas it spurred Mozilla on to improve it, so that it’s no longer bad like that.)
I look at the sessions of people that use Chrome, and I wonder how they ever manage to get anything done: tab management is uniformly hopeless. Firefox, on the other hand, copes with hundreds of tabs with perfect equanimity. (I currently have 165 tabs spread across three windows—one for personal stuff, one for work stuff, and one for a particular work project; this is a fairly typical number; I might have less than twenty once a year or so, and I’ve had as many as 450 before.)
I use the Tree Style Tab extension for a vertical tab bar with hierarchy. I’ve suggested it to family members once or twice each, and my dad and two of my sisters use vertical tabs (two via Tree Style Tab, one via some other extension) and wouldn’t be without ’em. Most of the rest of the family uses Firefox too, and I believe Firefox is the leading browser used by my coworkers at FastMail by quite a margin.
Firefox’s awesome bar is aptly named. It’s way more useful than Chrome’s.
I am an idealist and not fond of Google, and while that’s a factor, it’s not the primary reason I use Firefox. I use Firefox because I find it simply better. (Same deal with using DuckDuckGo over Google. It’s more usable with keyboard shortcuts and bangs, and finds results every bit as good as Google’s.)
I also use Firefox on my phone; there, it’s more of a mixed bag: it’s markedly slower than Chrome in general, but I can install regular extensions like uBlock Origin and Stylus. There, my dislike of Google is more of a factor.