Firefox, on Linux. Though I'd be genuinely surprised to hear the issue with scaling worked differently elsewhere.

Firefox is scaling its base size based on the screen, and then the page is scaled up further from there to have a readable font size. My base is 150% scaling, but the UI has comparably uneven strokes at many different scales and page sizes. (The strokes get less uneven the bigger they are, since at larger sizes there are more pixels to work with.)

Easy way to compare font scaling quality: At any scale, hitting one of the "change" links in the configuration menu brings up a browser UI element, using a font that looks great at any size.

Happy to file a report with the details.