"It's not fast, but the best at all the things you want to do." -- "So you should be using Chrome instead!"
To strengthen you point: if you want to use the same browser on desktop and on your smartphone, Firefox is the only one that has proper ad-blocking on mobile. Many other mobile browsers have ad-blocking, but none of the block those intolerable "related content" fake news bottom-of-the-page spammers. Only Firefox with ublock origin properly blocks invasive ads on Android.
The other ad-blockers on mobile (integrated in the browser or not) suck so much that I'm beginning to think they are all getting money from advertisers. And of course, google chrome does not block anything at all, which is very bad considered how much android malvertising there is. There is one Android browser called Yandex, that support desktop extensions, but surprise!, ublock and adblock plus are not supported. You are stuck with its built-in adblocker, which does not block anything.
I've tried so many times to use anything else that firefox on Android, but after two days of surfing the web in a minuscule square in the center of flashing advertisement, after being redirected to the play store to install stupid games, after too many pop-overs with a minuscule x, I always go back to Firefox.
To sum up: if Firefox is noticeably slow on Android, it is much less noticeably so on any decent desktop or laptop. Also, I like syncing desktop with mobile, to avoid having to retype password on the virtual keyboard, and to quickly find in the history a page I may have visited on my phone. So Firefox be it, on mobile because it is by far the best (even if very slow) and on desktop because it does not really suck that much, and it is the only one that sync with mobile.
But the day where Chrome or Opera support proper ad-blocking on Android (and Opera can do ffmpeg decode on Linux without going crazy), or Vivaldi releases an Android version, I'm sorry for Firefox, if it has not catch up on performance, its going to be retired.