Why does this website look terrible in Firefox?

When i “let pages choose their own font” in Firefox prefs, this site and many others’ default text renders horribly. Strangely, i haven’t found any clear, concise discussion or solutions for this – just a lot of complaints on various forums.

The sans-serif default font here is crooked, wiry, and has a lot of bleed. Arial and other fonts render fine, but the primary workarounds – override page font choices, turn on Windows’ Font Smoothing for the whole screen – are not options for various reasons (they break too many other things). Does anyone have a solid working solution to this irritating problem? And does anyone know why Mozilla has let this persist for so many years?