As mentioned in a post about my first impressions of the M10 FHD tablet, Firefox and other desktop apps look tiny on the 1920 x 1200 screen on the 10 inch tablet. The same problem occurs if you try to use Firefox on any computer with a high resolution screen. After writing that post, I realised that there might be an internal setting in Firefox for scaling. As is often the case, the Arch Linux wiki had the answer:
Open Firefox advanced preferences page (
about:config) and set parameter
2 (or find the one that suits you better;
2 is a good choice for Retina screens).
After some trial and error, I have found 1.7 to work well on the M10 FHD. The UI elements are large enough to be readable and quite a lot of content fit within the browser window with this scaling.