1) Multi-account containers - open each tab in an isolated container with its own session cookies etc. Use case? Login to multiple Google/Azure/AWS/etc accounts at the same time on different tabs. This is a game-changer if you have to login to multiple cloud accounts because you work for a multinational and each subsidiary you support is on a different tenant.
2) Firefox Lockwise - (though they are deprecating this on Dec 13) standalone password manager that syncs with Firefox's passwords, and which works on any iOS browser/password window. (I would use Keychain except I don't use Mac desktops).
There are some downsides:
1) Some plugins don't exist on Firefox (e.g. Language Learning with Netflix), and those that do don't compare well to their Chrome cousins (Google Translate -- doesn't do popup translations). Most mainstream plugins however work extremely well (uBlock Origin etc.)
2) Some pages don't render because webpage author targets Chrome (but this is very very very tiny % of pages that one would normally encounter -- I encounter them because of poor web authoring practices on the company intranet).
The render/Javascript performance is pretty darned good. Benchmarks say it is hair slower than Chrom(ium), but in practice, it's pretty snappy even on older hardware (I run Linux on a 2014 Dell) so I'd say it's a non-issue.