Reproduced the initial issue using latest Nightly 80.0a1. Verified that using Firefox 78.0.2 and Firefox 78.0.2esr when accessing the url received in outlook will correctly ask to open Teams Microsoft and then the app is opened/call is started across platforms (Windows 10 64bit, macOS 10.15.6 and Ubuntu 18.04 64bit). I also verified that flipping the security.allow_disjointed_external_uri_loads pref to true also fixes this in latest Nightly.
Bug 1650162 Comment 29 Edit History
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Reproduced the initial issue using latest Nightly 80.0a1. Verified that using Firefox 78.0.2 and Firefox 78.0.2esr when accessing the url received in outlook will correctly ask to open Teams Microsoft and then the app is opened/call is started across platforms (Windows 10 64bit, macOS 10.15.6 and Ubuntu 18.04 64bit). I also verified that flipping the `security.allow_disjointed_external_uri_loads` pref to `true` also fixes this in latest Nightly.