Complex sentence: FireGPG is an extension for Mozilla Firefox that allows the user to encrypt and decrypt text within the web browser using GnuPG, which must first be installed before firegpg can work.
Split into simpler sentences: Firegpg is an extension for Mozilla Firefox that allows the user to encrypt and decrypt text within the web browser. The text is encrypted and decrypted using gnupg software, which must first be installed before firegpg can work.