When Slack launched video calls, it was using Chrome's own implementation of WebRTC which is called Plan B. So it worked in both their app which is electron based and Chrome. Not sure if it worked with chromium derivatives like Brave, Vivaldi, etc.

Firefox rolled out the standard based WebRTC implementation and chrome eventually did as well, but companies like Slack and various others didn't migrate instead just threw an error message saying calling isn't supported in Firefox and instead use their app or chrome.

Since Slack's partnership with AWS about 2 years ago, they migrated from their own call infrastructure to using AWS's Chime. Chime SDK has support for all major browsers https://docs.aws.amazon.com/chime/latest/dg/meetings-sdk.htm...

But their browser check still remains today and Mozilla started to spoof the user agent as a workaround so that Firefox users can make calls as Slack aren't willing to remove the check or properly add support.

Chrome is planning to remove WebRTC plan B soon, so this should force companies to use the unified WebRTC implementation and hopefully means Firefox support. https://groups.google.com/a/chromium.org/g/blink-dev/c/IY2am...