Earlier quoted context omitted.
Chromium isn't, either. But the extension flexibility was one of the major points for me. I'll have to pin Firefox to 56.x in my package manager.
That's a pretty terrible idea. Your web browser is probably the most important piece of software to keep up to date, besides the OS for security reasons.
No browser on Linux can handle different DPI per screen. For Firefox, there's an addon implementing that today, but it won't work with WebExtensions.
So I have to use today's Firefox - or I can't use any browser.
Well, there's also the third option of using a magnifying glass to read websites.