Thanks for the report. You were right that Android was not working correctly in version 4.9. I investigated it and fixed the issue in version 4.10.

Version 4.10 has been submitted to Mozilla and is waiting for review/publication.

One important detail: Clear Cache clears Firefox browsing data through Firefox’s WebExtension API. It does not clear the Android system-level app cache shown in the phone’s App Settings screen, so that number may not change even when Firefox’s web cache is cleared.

Once 4.10 is available, please update and try again.