I just wrote a little script to fix it temporarily until the next version comes out:
Yours may work for you, but it anyone else using your script should know that the above code will break their other ON HOTLINK events, if they have any. You have to use the same check in both hotlink events to make sure it will only trigger for that hotlink, otherwise you'll be running firefox a lot more than you need to.
A slightly improved version is the following, which also allows for https/ftps links and removes any trailing commas, periods or exclamation points people tend to use when linking URLs (Jonny says: check out this site http://yahoo.com!). It works pretty much like mIRC's internal one:
on $*:HOTLINK:/^\W*(((?:https?|ftps?):\/\/|www\.)\S+)\b/:*:run firefox.exe -new-tab $regml(1)
edited: absolute path to program files is not needed in vista, thanks steven!
edited again: links now load in tabs if your browser is already open (we all love tabs don't we? if you don't, remove the '-new-tab' argument)