Seems like this should be configurable, but I've not found a way to do it. Interested in knowing if the y! toolbar does this, but even more interested in learning how to do it without adding that.
Answers:
If you mean that you want links that you get sent in Yahoo messenger to open in Firefox, then the option is under Options in Firefox... at least it is for Windows, I'd imagine it would be similar for Macs. First you'd have to make sure that Firefox is your default browser, then you can decide whether the links open in new windows or new tabs. In Yahoo, you just have to make sure under the General tab in Preferences, the setting for "Open web links in a new browser window" is checked.
If this is not what you're looking for then I'll go take a long walk off a short pier. :)
u want to make firefox ur default browser.. then everything shd work fine..