Same! Had Developer edition installed for testing, but switched fully today (Uninstalled Developer, installed Nightly). The fonts look nice, it's got wayland support on my Linux partition (on Fedora), and it's just really pleasant!
I noticed this too in FF on Linux, I inspect elemented and for some reason a huge number of websites were using Nimbus Sans as the font... Later when I eventually used someone's Windows install I realized that Nimbus was only being used on my computer... Wtf?
It's a font developed long ago by some famous font foundry, and a subset covering common characters was made free and included in Linux printer drivers. It is visually very similar to Helvetica/San Francisco. I installed GNU FreeFont (probably available in your distro's repos) which looks the exact same but covers many more characters than Nimbus. In Firefox I went to the font config settings, set FreeSans/FreeSerif as the default fonts, and unchecked the option to allow websites to specify custom fonts. Just don't disable the checkbox to download web fonts, or icon fonts with various icons/glyphs will show as boxes.
It's truly beautiful - every once in a while I see some text and just stop reading and stare at it for a few seconds. On the rare occasion when I go to a website about fonts I use another browser to be able to see their CSS-specified fonts.
Got a new laptop and that in addition to Google's evergrowing awfulness prompted the switch. Was pleasantly surprised that my u2f tokens worked as expected.
Ah how come? It's being great to me so far! Back before the Chrome days, '00s and early '10s, every geek like me used to use Nightly, so I went with that.
