BXE doesn't work on Firefox 1.0.3 and Mozilla 1.7.7, due to a ā€œbadlyā€ implemented security fix.

The cause of the problem is described here:

bugzilla.mozilla.org/show_bug.cgi

So, blame the firefox QA team ;) Or in the words of Brendan Eich:

Great. Law of Unintended Consequences strikes. We didn't test. The patch is one-liner.

Basically, all ā€œuserlandā€ javascripts, which use prototype to overwrite/add getters/setters to native objects are broken currently. There's a ā€œsolutionā€ described there, but I'm not sure if i'm gonna

apply that. I have tons of prototype.defineGetters in BXEā€¦

If I'd only know, when they release 1.0.4ā€¦

Update: Until 1.0.4 is officially released, you can get one of the nightlies from the mozilla server, which have the bug fixed.