When I switched from FF to WF it was lighter on system resources, faster and cut out a lot of the telemetry included in FF.
I’ve not used Firefox for well over a year now, so I can’t speak to that still being the case, but those were the main draws for me.
Stability – So, uptime and rate at which it updates with Lemmy.
Moderation – One or two good Admins at least to keep things running smoothly and communicate with us Lemmings effectively, and have a decent ruleset.
Federation – Wide enough that I can get the content I’m looking for whilst (likely) not dealing with nutjobs.
The rest is all good, the federated nature of everything means I don’t really need to be where the communities I use the most are.