Useragent interpretation Bug
outlook.office365.com does not interprete the useragent correctly. Using a current Firefox on FreeBSD redirects to the old site. Faking the Operating system to state this is Linux and Outlook works.
A currentf Firefox Browser is independent of the Opperating system. All other office365 apps work, but outlook does the incomplete and invalid user agent detection. Please accept Firefox as a valid browser on all Plattforms as done on All other Microsoft webapps. This is a Bug.

2 comments
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Thomas Zander commented
Follow-up: As long as a supported user agent string is found somewhere in the OS identifier section, it works fine. I tested with 'Mozilla/5.0 (X11; FreeBSD amd64; Linux x86_64 compatible; rv:80.0) Gecko/20100101 Firefox/80.0'. This works too.
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Thomas Zander commented
Confirmed. This bug is still not resolved.
Steps to verify that the full-featured HTML5 UI indeed works on Firefox on FreeBSD.
In "about:config" set the variable 'general.useragent.override' to a different OS, for example: 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:80.0) Gecko/20100101 Firefox/80.0'. Then reload the page. Outlook will load the full-featured UI and everything will work just fine.
This suggests that actually everything is fine with the service itself, and that just the user agent evaluation is a bit too restrictive.