Someone wrote a comment recently (Sep 25, 2024) on this 5-year-old issue in the Mozilla bug tracker:
[meta] Firefox for Librem 5, PinePhone & other Linux phones support tracker
the setting page is getting redesigned and is confirmed to be responsive: https://connect.mozilla.org/t5/discussions/firefox-settings-design-share-your-input/m-p/66629/highlight/true#M23625
This is an answer to settings page, not to Firefox in general. Here you can see a very early design. However, these are also good news since other parts may follow up.
The design just looks like they want to push the hamburger menu even further, which I really really dislike (desktop and mobile!). There are disadvantages for such menus on complex software as Firefox is.
There seems to be some good progress here, linked from the Mozilla bug tracker:
