For what it's worth, I've checked this on my Galaxy S20 in Chrome and Firefox (87 rc1, not the beta 88) and no problem getting the notifications. FF was a fresh install from the play store and I took the most 'locked-down' options in the first run setup, apart from defaulting to incognito.
Trying the same thing on a Galaxy S7 that I haven't got around to recycling yet (!); I got the notifications for the FF browser but not, interestingly, Chrome. Until, that is, I went in to the 'Settings/Notifications' on the S7 and found that notifications from Chrome were disabled under 'APP NOTIFICATIONS'. Enable that, and hey presto - get Groups.io notifications in both FF and Chrome.
In summary; looks like it's down to individual 'phone settings rather than anything on the Groups.io side