Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
Most Emit1 call sites only use the result to test for success/failure.
It's error-prone. Out param for the call sites that need it.
While we're on the subject, these should probably be methods of BytecodeEmitter.