Thanks to Steve Babineau’s help, we’ve tracked down the error to a conflict with (at least) one other Firefox extension (Server Spy, in his case). Any extension which declares its own “gBrowser” object (overriding the built-in Firefox variable) would cause this problem.

A test build working around this problem is attached to the bug in Bugzilla. If all goes well, this will be released as v1.3.7 within a day.