You can't really have usable vertical tabs in Chromium via plugins either, unless you're content with wasting a lot of horizontal space for an ugly sidebar and vertical space for uselessly duplicated tab bar.
Firefox is the only actual choice I'm aware about.
Why? Are you considering forking Firefox?
I don’t see how just using it does. So if you aren’t contributing to it you may as well use the browser with the best feature set for your use case.
lower market share =>
nobody testing against the free browser or fixing site breakage =>
quirks (bugs, underdefined specifications, nonstandard features) of other browsers becoming required for a functional Web =>
free browser is no longer a browser of the actual Web.I don’t see how that relates to market share, since regular users won’t do that.