I use Firefox (desktop) on mine and it works quite well, and has good performance. I wonder if it can be configured to be a bit more mobile-friendly OOTB, maybe with some about:config flags?
There's a meta-bug on Bugzilla for adding a mobile-style frontend to desktop Firefox for the Librem 5 and Pinephone: https://bugzilla.mozilla.org/show_bug.cgi?id=1579348 Not a lot of work has been done since hardware has been scarce so it has not been a high priority.
