Legal considerations are important here. Even though it's an emulator, users should have their own ROMs unless from public domain. Mention that hosting ROMs is illegal, so focus on the app's legality.

Potential issues: The 2021 version might have bugs or lack updates. Mediafire links can change, so advising users to check for the latest version. Also, some antivirus might flag emulator APKs as malicious due to their behavior, so include notes about that.

Also, check if there are alternatives like AndroZen or other emulators that might be better or updated. Since the user is asking about FBA4Droid specifically, focus on that, but a brief mention of alternatives could help them decide.

I should also mention that Mediafire links can sometimes be outdated or removed, so advising users to verify the source or look for official repositories if possible. But since the user specifically mentioned Mediafire, focus on that method.

I need to ensure the post is informative but not just a download link. Users might be looking for a reliable way to set up the emulator. Steps to download from Mediafire include installing Mediafire app, searching for "FBA4Droid 2021," downloading the APK, enabling unknown sources, and then installation. Usage tips: where to place ROMs, how the UI works, maybe some controls setup.

In conclusion, the post should guide the user through downloading, installing, using the emulator, while emphasizing legal and ethical use. Make sure it's clear that the app itself is legal and that the post doesn't host or provide ROMs.