To change the default browser in Outlook 365, go to File > Options > Advanced, then scroll to “Link Handling” and set “Open hyperlinks from Outlook in” to your preferred browser, like Chrome.
I thought there was a way to change the default web browser in mIRC, or at least that there used to be. I am using v6.03. The search was broken last night when I started my FAQ thread (I had to hunt ...