3.5 rc1 seems to run firebug (1.4.0 b2) just fine.
The only issue I have run into is that one time I managed to crash
firefox with this html:
<html><head><title>test</title></head><body><a href="#"
onclick="location.go(-1)">go back</a></body>
by clicking on the break on next statement button and then clicking on
the link (note that it is invalid javascript calling location.go
instead of history.go; location is undefined). I think that I was
reading somewhere that this was a bug that was already fixed in
development versions though(hence I didn't bother reporting it;
besides the fact that I only managed to make it happen one time).