The best workaround I've found is to run multiple instances of Firefox concurrently, one for each MS Teams tenant you need to use. See here: https://www.filecritic.com/how-to-run-multiple-instances-of-firefox-at-the-same-time/
Microsoft surely must be working on allowing connections to multiple tenants through Teams. Not having that feature is surely preventing adoption of Teams to some degree, and I sure can't recommend Teams to any of my other clients until this is added.