(In reply to imthepipe from comment #17)
I have screenshots to show the difference, but for the life of me can't see (no pun intended) how to attach/upload them.
You can upload files here: https://bugzilla.mozilla.org/attachment.cgi?bugid=1578454&action=enter
You'll have to use the form more than once for multiple screenshots.
But it sounds like you used a combination of full page zoom and altering the reader mode font size, in Firefox 68 and earlier. We changed reader mode so now, zoom shortcuts no longer use full page zoom on reader mode and only change the font size. This was felt to be an advantage because this way, the reader mode UI does not change, only the content - just like for other pages, the browser UI does not zoom, only the content. There isn't another practical way to achieve that effect without "rerouting" what the page zoom shortcuts do.
I'm a little surprised here, because the changed font size should now be persisted - that is, once you've increased the font size / zoom level, it should stay zoomed in anyway, and the same font size / zoom level should apply on other things you view in reader mode. Whereas presumably you're executing this bit of JS every time you open reader mode...
Another related issue is that the zoom level shown in the FF menu while in reader mode is inaccurate. It doesn't change when zooming, nor does it display the correct zoom level after zooming.
Right, because the zoom indicators are based on full page zoom, which in fact is not changing while you're using the shortcuts (or any of the other UI entrypoints, for that matter).
What do the indicators tell you for reader mode if you always have it zoomed in, though? That is, does this really matter, or was it just an observation that you hoped was helpful? (That's fine, just trying to make sure I'm not missing something else.) :-)