The comments in https://superuser.com/a/370235/478867 read:
This recently just happened to me, after over 15 years of using it.
Changing network.protocol-handler.external.mailto to false seems to
have worked for me. – Asinine Sep 21 '20 at 21:12
This did the trick for outlook365 and FF 89. – Alfabravo Jul 7 at 14:01
I am using ff 89 and when I changed network.protocol-handler.external.mailto to false in about:config, the pop-up doesn't come up anymore when I refresh the page.
It still works for ff 124.
The drawback of this solution is that any association with mailto links stops working too.