Back button not working in Firefox:

• The majority of links you click open in the same browser tab. However, if the Back button on a newly loaded page is greyed out, it's because it was launched in a new tab or window. You won't be able to utilise the Back button in such situation. Switching tabs or windows is the only option to return to the previous page.
• If the Back button appears, make a point of uninstalling any apps that involve gestures. Custom launchers, custom navigation apps, and apps with a permanent overlay on the Home Screen are all examples of this. However, if your Android's Back button is still not working, try deleting the cache sector.

Read More : How do I fix the 'manage tax info' issue in Google AdSense?