http://www.mozilla.org/projects/xul/xre.html

"The first goal is to be able to build stand alone, profile independent, XUL based applications from the mozilla trunk. Current examples include the Mozilla application suite, Firebird, and Thunderbird.

The second goal would be to have all these application share and use a XRE, just like Gecko based applications will be able to share and use a GRE.

We are starting with the first goal, while we let the GRE pave the way for the second goal."

So it looks like any independent XUL based application, including Mozilla Browser (Firebird), will be able to use XRE.