Comment 3 for bug 750556

I've no idea. You need to report extension bugs to whoever provides you the extension. In any case, if if you have downloaded an xpi from addons.mozilla.org and it depends on libnotify-bin, then there's not much the extension author can do about that other than tell you to install it (xpi's have no way to declare dependencies on distro-packages).

I'm guessing you are using something like https://addons.mozilla.org/af/thunderbird/addon/gnome-integration/