After some tinkering with two different local installations, I think I have been able to pinpoint the culprit for my problem with your theme.
I applied it to a working version 1.5 and indeed everything was just the way I expected it: nice screenwide breadcrumb trail. I made some changes to the header file and it all looked splendid. Until I upgraded to version 1.5+ (or moodle_15_latest). And there it was again: a cut short breadcrumb line.
So it seems to me that somewhere in that "development version" there are changes since the official 1.5 version that cause this unelegant behaviour.
BTW, to answer your previous posting: my default browser is
Firefox and I always check things in
IE 6, and both gave similar results. But from what I wrote above, I tend to conclude the cause of the problem is related to 1.5+
This does make me worry a bit about what will happen in the upcoming 1.5.1 release. I expect the same...