Out of those who actually give a reason for this position, which is practically no one, it's almost always "vertical tabs". Apparently those people can't use a search engine since there are multiple easily found extensions available that make FF do vertical tabs.

[–] HKayn@dormi.zone 1 points 1 year ago

Then how about tab tiling, multi-level tab bars, workspaces, automatic session backups, sidebar panels, Spotlight-like global search, commands to toggle parts of the UI and custom toolbar buttons?

I can probably find an extension for most of these if I look hard enough, but then I'd have to worry about them being maintained and not sold out. At that point I'd rather use a browser that has all this built-in.