Firefox (at least the old one) is the only browser I have found that decently runs the HTML Validator extension (with View Source and no crashing). This is now the only thing I use Firefox for. Since I only need to validate pages I have made myself, security updates are not an issue; so it probably doesn't matter if it eventually goes to seed.
I use Waterfox and have bogged it down with add-ons. This Mozilla release has, if nothing else, reminded me how fast a browser can be, but I’m still not discarding my well armored Waterfox. I use different browsers for different purposes.