Most of the third world leaped the desktop stage and are coming online using mobile. Primarily due to cost factor but also the additional infrastructure (telephone line, router, dongle etc) and maintenance (dirt/dust on keyboards) you need for a desktop. And imho, this trend will catch up in the first world countries specially with newer generations.
Hence I would think focusing on mobile would be the best strategy for FF to gain market share. Additionally mobile users are more sensitive to data usage. A lot of data is consumed by downloading tracking scripts and serving rich content ads. Currently there are no viable alternatives as we can't install uBlock or other extension for Mobile browser. If these features ships out of the box, I'm pretty sure it would increase adoption purely by word-of-mouth. The case I can think of when this strategy would not work is if users on mobile are spending most of their time on individual apps (Google assistant, Siri, Fb/Instagram/Twitter mobile app) and not on browser.
Which brings me to the state of Firefox mobile app. Firefox's mobile market share [0] is non -existent compared to desktop [1], which was at 32% at one point in time. I may be a minority here (HN/US), as I do most of my browsing on the phone. (I use desktop only for searching of technical implementation/issues. Primarily because this way I'm aware of the cost and duration that I've been browsing as I would have to reach for my phone and divert my attention away from desktop.) I tried to switch to Firefox mobile app on android but it felt like a desktop app jammed into a mobile viewport. A lot of space is devoted to the tab and address bar. A lot of sites that I navigate to, renders a desktop page instead of triggering a mobile layout as they do if I were to access it through Chrome. All of this results in a bad UX.
IMHO, focusing on Firefox mobile app's performance, privacy and usability will help grow adoption of Firefox overall, specially in the environment where discussion on privacy are becoming increasingly mainstream.
0: https://gs.statcounter.com/browser-market-share/mobile/world... 1: https://gs.statcounter.com/browser-market-share/desktop/worl...