(1623340) Firefox compatiblity with Microsoft Teams

User agent overrides

Spun off from https://bugzilla.mozilla.org/show_bug.cgi?id=1623340#c5

A fix would be for Firefox to spoof its own user-agent string, so it represents itself as whatever the latest version of Edge is.

The functionality works, but Microsoft is blocking it through user-agent sniffing. …

Firefox could spoof the user-agent when any Microsoft site is visited (based on a list of primary domains, perhaps). …

… In addition to the HTTP User-Agent, changes may also be needed to data in the window.navigator object to circumvent this.

Earlier this week I began testing this Edge string (the latest):

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.102 Safari/537.36 Edg/85.0.564.51

– overriding these two URLs:


– specified in Custom UserAgent String added to Firefox 80.0.1 on FreeBSD-CURRENT.


Meet in a group context: a moment after apparently beginning the meeting – the camera image appears as it should – the image disappears, leaving an alert:

Sorry, we couldn’t connect you.

– and two buttons:

  • Retry
  • Dismiss

– retries do not succeed.

Unwanted effects of the override include breakage of OneNote:

Sorry, we ran into a problem.

Your feedback helps Microsoft improve OneNote. Give feedback to Microsoft

  • Restart

– restarts lead to the same problem.

Without the override for the two domains

OneNote works:

With the override for https://teams.microsoft.com/ alone

OneNote breaks.

With the override for https://statics.teams.cdn.office.net/ alone

OneNote works.

Meet in a group context results in an explicit alert, before the AV dialogue:

Use the app or a supported browser


any news on this issue?
Thank you

On the Mozilla side: none that I can see.

On the Microsoft side: