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.
