Change storage.sync to storage.local, the user will have to set their region in every browser instance (not session)
2 manifest files - manifest-$browser.json and 2 release assets. However during development and testing - it is not easy to test the extension between browsers without renaming the files to manifest.json
browser_specific_settings is not supported in Chrome manifest
See this for compatibility information: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/browser_specific_settings
This key is required for storage.sync on Firefox: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage
Potential solutions: