Manage Firefox preferences through `user.js` instead of `prefs.js`.
Because of Mozilla's recommendation against direct `prefs.js` edits to
avoid potential profile corruption. Instead, the `user.js` file, if
present, overrides the settings in `prefs.js` at application startup.
Change AddFirefoxPrefs function to update `user.js` and manage
creation/deletion of this file:
1. Handle file creation if `user.js` does not exist.
2. Deletes file if `user.js` becomes empty after reverting settings.
Other changes:
- Improve log messages
- Minimal refactorings
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