When I run the latest firefox, I get the following message:

	Extension System Warning: Failed to set up default extensions
	files probably because you do not have write privileges to
	this location. While you can run Firefox like this, it is
	recommended that you run it at least once with privileges
	that allow it to generate these initial files to improve
	start performance. Running from a disk image on MacOS X is
	not recommended.*** loading the extensions datasource

(or rather, I get the same thing without *any* line breaks, as if it's
assuming some pop-up box for errors, rather than an xterm...)  But
I have no idea what it's referring to, and it takes long enough to
fire up that I'm afraid to even think about running it under ktrace.
Grepping the source tree was not fruitful.