What are you doing that brings up Edge? Firefox may be default in your heart, but sounds like it's not set on the computer.  ;-)) Just click the Windows key, start typing Default and you should see Default Apps. Click that and see what's set as default browser.

Also check the settings in Firefox to see if it's set as Default. see screenshot

I have this problem too. Firefox settings show Firefox as default browser, Windows settings "default apps" also show: Web Browser Firefox but everytime I open an email it goes to Edge. .

Microsoft Outlook 365 has its own default browser setting.