The font scaling was available in the previous Firefox on Android (i.e. ≤ 68), too, but it had been disabled by default for a long time because while it does make text more readable on pages written without mobile phones in mind, it can also cause some layout breakage on some pages, and there was a little tug-of-war between people preferring the former even at the cost of some possible layout breakage, and those wanting to avoid the latter even at the cost of unreadable text. If you knew about it, it could still be re-enabled through the regular settings, though.

After having fixed a few bugs in that regard, I pushed for giving re-enabling it by default another try with the rewritten browser, and so far that decision luckily (from my point of view) seems to have stuck.

Additionally, it has recently turned out that for pages specifying an explicit desktop-sized viewport (i.e something like meta name="viewport" content="width=1024", as opposed to either using nothing at all, which gives the standard desktop-size viewport of 980 px, or "width=device-width", meaning it's a mobile-friendly responsive layout), there was a long-standing bug meaning that the font scaling for desktop-style pages was erroneously being deactivated on xxhdpi-phones.

This latter bug affects the desktop versions of both Reddit and Slashdot for example, as both of those are using an explicitly sized viewport. It has now been fixed in Firefox 93 (https://bugzilla.mozilla.org/show_bug.cgi?id=1685756)