I primarily use Chrome just for the reason that it has better UI than Firefox on Linux. The developer tools on Firefox is too ugly, they definitely need a more compact design.
You can see some mockups on Victoria Wang's Twitter: https://twitter.com/violasong/status/926189788486557697
http://www.omgubuntu.co.uk/2017/08/firefox-client-side-decor...
The icon indicators also are really nice. At this point the only reason I use Chrome is b/c of Google Meet (which doesn’t support Firefox).
The only thing I wish was better in the beta is battery life/usage. Safari is still king in terms of its effect on you battery.
Edit: to be clear I use Firefox beta on Linux and macOS regularly.