Brave blocks ads natively in their mobile browser, you should give them a shot.
Why not Firefox? Bad redesign release a few years back and now discernible way to change the accessibility so it holds always. Old.reddit text is too small to be read no matter what I've tried.
None of the other browsers on Android I've found to be worth the time.
don’t know about Brave but μblock origin fails for this site on Firefox mobile on iOS
uBlock Origin with Firefox on the desktop seems to work for me, though there probably are a few differences between mobile and desktop.
Firefox (and other browsers) on iOS are pretty limited in what they are allowed to do. It is quite likely Admiral is able to detect that easily compared to more robust solutions on Android.