Haven't followed the mobile browser scene so correct me if things have changed, but the main draw for FF on mobile (at least on Android) has been:

- Autoplay in the background without YouTube Premium - Extensions - specifically uBlock Origin, making it an option (the only?) for ad blocking on mobile. - Sync your FF data between desktops and mobile - great if I need to access something on the go without a password manager.

I hate its sluggishness as well but the pros are pretty hefty IMO.

For those using F-Droid, there's also Bromite, an ungoogled Chromium with ad blocking built in. Haven't used it too much.