Put addon on Firefox repository

[tobiasdiez]

Update the package meta data (for example, the url field) and make it possible to Install the addon from the Firefox central repository (submit here). Also close JabRef feature request.

[tobiasdiez]

The addon is now available under https://addons.mozilla.org/en-US/firefox/addon/jabfox/ and should be signed in around 10 days from now. I leave this issue open until the review my Mozilla is finished.

[tobiasdiez]

The addon is now signed and can be installed directly via https://addons.mozilla.org/en-US/firefox/addon/jabfox/.