Is Firefox set as your [[Make Firefox your default browser|default browser?]]

Yes it is

It seems like Microsoft might be sneakily forcing links to open in Edge in the Outlook app by default; bypassing the default app preference. Can you try following the [https://support.microsoft.com/en-us/topic/outlook-emails-open-next-to-web-links-in-microsoft-edge-b0e1a1c1-bd62-462c-9ed5-5938b9c649f0 instructions Microsoft have provided] (under 'I want to open links with the browser set as the default in Windows settings' in the FAQ section)?