Chrome is really popular. I'm pretty sure everyone knows it. Chart confirmation. I still use Firefox.
A good side tabs extension is the primary reason I use Firefox. A row of tabs on top is the worst thing ever. Lists are great:
I use the "Tab Tree" extension (thank you Sergey Zelentsov). Chrome has a handful of side tabs extensions, but I don't consider them usable.
Side tabs is the only reason I use Firefox.
Since I'm using Firefox on my laptop, I'd like everything to sync to my phone so I don't have to retype all my passwords. I like Firefox for Android's tab screen better than Chrome. That's the only difference I notice so that works out pretty well.
I'm in the dev tools quite a bit throughout the day, and I find the Firefox dev tools more intuitive. Chrome can take it's 'Elements', 'Sources', 'Application' tabs and shove it - I prefer 'Inspector', 'Debugger', 'Storage'. Honestly it's not a big difference to me. Some people feel strongly about dev tools.
Should I ever be debugging a cross-browser issue, my other (more popular) browsers are clean installs. Nice to have.
Firefox doesn't support integrated windows authentication. That's alright by me.
I don't understand why more people don't demand side tabs.