wow, old topic :P in case anyone's interested, and since i already had something similar lying around, here's a version of that snippet that more accurately reflects mIRC's own detection of URLs:

Code:
on ^*:hotlink:*:*:{
  if ($mouse.key !& 1) || (!$regex(url, $1, $urlre)) halt
}

on *:hotlink:*:*:{
  run "C:\Program Files\Mozilla Firefox\firefox.exe" $regml(url, 1)
}

alias urlre {
  return /(*UTF8) (?:^|[\ \x09]) [\Q '<([{" \E]* \K (                          $&
    (?: (?: (?# irc|gopher|telnet| ) https?|ftps?):// 	| (?:www|ftp)\. )      $&
    [^\ \x09]+\.[^\ \x09]+? ) (?= [\Q ')]}>"., \E]* (?:[\ \x09]|$) ) /giSx
}

"The only excuse for making a useless script is that one admires it intensely" - Oscar Wilde