I have been searching for a software replacement for fireGPG, a GnuPG tool management for Firefox to manage GnuPG operations/management. The project was discontinued three years ago because the author didn't have time to continue following GMail's changes.
Today, i found the alternative and that is WebPG. It's available for Firefox and Chrome. It's actively developed in github repository (Firefox and Chrome) by Kyle L. Huff. It still has some issues, but for most cases, it's not completely their fault and some of them are GMail's fault.
Let me know if you like this project :)