What's your desktop app version?
As for email notifications, there's a button in the Preferences menu that leads you to this page: https://clojurians.slack.com/account/notifications But it exists only on the web, so not logging in in your browser is not an option with Slack.
Also, this Slack—Clojurians—is on a sponsored (pro) plan so all the 10+ years of Clojurians history is available here for searching.
My version is the same, but from Snap. So maybe the Flatpak version is misbehaving with those "update your browser" messages.
Maybe your FF is outdated, or there's some user agent override, or something else. I've used Slack from FF just fine on a rare occasion. Flatpak and Snap packages can sometimes be problematic when they are misconfigured by their maintainers. And given the amount of options, it's very easy to misconfigure something.
We have an #C0B7Q4XRM1U channel which should be used for any follow-up discussion. (There's also #C068E9L5M2Q if you need/want that).
ooo thank you