I understand that data is not cleared, you may need a file explorer app in order to navigate to the data. This should clear this data as well as an uninstall.

• Clear your browsing history and other personal data on Firefox for Android

Unfortunately, all of the data and pref files are installed in the system directory and are not accessible without rooting. Why they chose to do this is beyond me. I have my htc one rooted but not my tablet. When looking at health report it shows changes were made to the file by something else.

Where exactly is the file(s) you are referencing stored? Maybe clearing this from my rooted devices might fix the issue I've had for a few weeks (posted in another ticket).

Its under system root\data\data\org.mozilla.firefox(beta)\files\mozilla\ here you will see the profile folder xxxx.default. This is where its located. If you want a true clean install, uninstall Firefox then delete the org.mozilla.firefox. Then reinstall.