In my experience the PWA works fine on Chromium-based engines, but Firefox and Javascript-heavy applications has always been a suboptimal combination in my experience. Doubly so if you run any extensions (Dark Ready slowed my phone down to a crawl and uBlock Origin isn’t exactly light on resources either as far as I can tell) or have things like resistFingerprinting enabled in your about:config.

I used to prefer Bromite as a Google-free Chrome alternative, but that didn’t get updated in a while so I uninstalled it. These days I use Brave, that seems to be the browser with the best support without privacy infringements.

If the browser isn’t the problem and your phone just can’t run the web UI well, switching to Jerboa/Mlem seems like the only option. I’m personally watching the tafkars project with great interest, as that will hopefully allow some Reddit clients to be used on Lemmy servers once it’s feature complete.