Its Because you need Extract the patch from the Rar Folder. so read this Step by Step
How to Patch ROMS on an Android Device
1. Download the patch in the 1st post (if Some Rom Patches Appear to be in a Rar Folder like this (Ex.DR2.rar) This what simple RAR opener is For)
2. Go to the Google Play Store app On your Android Device , Look up Some apps called Unipatcher (a Rom Patcher) , My Boy (a GBA Emulator) And Simple Rar Opener (a RAR Opener).
{Notice : These are all FREE Apps on the Play Store}
3. While your Downloading the Apps, Google Pokemon Firered Rom USA on your Device Browser (I recommend Coolrom <dot> com )
4. Extract the Pokemon rom in a folder (Note : Create A Folder for Gba Roms And )
5. Open Unipatcher, select patch and the rom, then tap the save icon and the Patched Rom will be in the same Folder As the The Clean Rom
6. Rename the Patched rom With your Android Device's File Manager(Ex. Pokemon - DarkViolet Beta2)
7. and Done Just Play it with Gba Emulator