This is still under review by the feature team. No new details yet.
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The latest Version of Seamonkey 2.46 is locked out of Teams whilst it works just fine with the rest of O365/SPO (under Windows 7 and LinuxMint 13 and 18/Lubuntu 16.04)
SeaMonkey is basically just Firefox wrapped in a more traditional UI so underneath it is just as capable as Firefox is.
Vivaldi and Opera area also locked out and these are both based on Chromium so have pretty much the same feature set as Chrome.