Ha, this website seems to trip up the WebKit layout code (https://bugs.webkit.org/show_bug.cgi?id=211679) and cause large spaces to appear randomly in the middle of paragraphs that go away when you select the text…
We had noticed this early on indeed, but no idea what's going on. Thank you for reporting it — we didn't think of raising it against WebKit since we assumed they'd be Doing The Right Thing™.
They both seem to do some kind of anti-aliasing on a hidpi screen for me, and Firefox is actually slightly sharper here. The screenshot is nicely pixelated and somewhat easier to read, but the font itself on the page is a bit blurry.
Kerning seems to be very browser dependent as well, seems like Blink does better job there.