- Return early in `PDFViewerApplication._initializeJavaScript` for PDF documents without any `fieldObjects`, which is the vast majority of all documents, to prevent errors when trying to parse a non-existent object.
- Similar to the other `PDFViewerApplication._initialize*` methods, ignore the `fieldObjects` if the document was closed before the data resolved.
- Fix the JSDoc comment for the `generateRandomStringForSandbox` helper function, since there's currently a bit too much copy-and-paste going on :-)
- Change `FirefoxScripting` to a class with static methods, which is consistent with the surrounding code in `web/firefoxcom.js`.