I can’t speak for Slack, but I ship Mailspring for Linux as a Snap (and not a Flatpak) because snap provides a unified approach to automatic updates and the app is updated frequently. It’s an a major win from a developer perspective, because users that have to install the new deb / rpm / flatpak manually are almost certainly going to fall behind and then file bugs / have issues that take up time to triage.

(I know there’s a way to do autoupdate with Flatpak, but it is not tested and maintained as part of Flatpak itself, and when I looked it seemed to be a proof of concept with no production use!)

Slack is packaged as flatpak on flathub: https://flathub.org/apps.html

Various versions of firefox as flatpak are here: https://firefox-flatpak.mojefedora.cz/

Getting some of those on flathub is a work in progress.

There has been a Firefox Flatpak for a long time: https://firefox-flatpak.mojefedora.cz/

And there has been discussions about getting an official one from Mozilla.