It's up to the browser how it'll render your HTML - it's another case of how coding for IE can lead you into problems with other browsers. Digging through the RFC's I cannot find a standard for linebreak characters in ALT attributes (unless someone can enlighten me), but I have seen some proposition for one...
If there was a &xx; version of <br>, that would be defined as supported, but there is no such thing as &BR; - lobby for it if you want it included in the standards!
This is more of a quirk in IE than a problem with firefox.
I prefer the word "feature" . However, I understand that the functionality of what I was doing is propietory... another MS bending of rules if you will.