The exact thing that happens (as far as I can judge), is that my mIRC 6.16 opens URLs with Internet Explorer.

It does so when I want to open an URL:
  • /url (scripted or typed by hand)
  • /url -n (scripted or typed by hand)
  • doubleclicking an URL in a channel or query window
  • doubleclicking an URL in the catcher window


It opens URLs with Firefox however when I use /run, in which case it displays
Code:
* /run: unable to open 'www.test.com'
Despite the error, the URL opens just fine, in Firefox.

I did check my Firefox settings and it is undoubtedly my default browser.