Restore Firefox addons button

This updates the image URL to the current location
(from https://blog.mozilla.org/addons/2015/11/10/promote-your-add-ons-with-the-get-the-add-on-button/)
though I have a feeling it may not be a permanent link either.

Author: ferdnyc

README.md

@@ -18,7 +18,7 @@ With GSConnect you can securely connect to mobile devices and other desktops to:
 Please see the **[Wiki][wiki]** for more information about
 **[Features][features]** and **[Help][help]**.
 
-[<img src="https://raw.githubusercontent.com/andyholmes/gnome-shell-extensions-badge/master/get-it-on-ego.svg?sanitize=true" alt="Get it on GNOME Extensions" height="100" align="middle">][ego] [<img alt="Available in the Chrome Web Store" src="https://storage.googleapis.com/chrome-gcs-uploader.appspot.com/image/WlD8wC6g8khYWPJUsQceQkhXSlv1/UV4C4ybeBTsZt43U4xis.png" align="middle" hspace="12"/>][chrome] [<img src="https://addons.cdn.mozilla.net/static/img/addons-buttons/AMO-button_1.png" alt="Get the Add-On" align="middle">][firefox]
+[<img src="https://raw.githubusercontent.com/andyholmes/gnome-shell-extensions-badge/master/get-it-on-ego.svg?sanitize=true" alt="Get it on GNOME Extensions" height="100" align="middle">][ego] [<img alt="Available in the Chrome Web Store" src="https://storage.googleapis.com/chrome-gcs-uploader.appspot.com/image/WlD8wC6g8khYWPJUsQceQkhXSlv1/UV4C4ybeBTsZt43U4xis.png" align="middle" hspace="12"/>][chrome] [<img src="https://ffp4g1ylyit3jdyti1hqcvtb-wpengine.netdna-ssl.com/addons/files/2015/11/get-the-addon.png" alt="Get the Add-On" align="middle">][firefox]
 
 [ego]: https://extensions.gnome.org/extension/1319/gsconnect/
 [chrome]: https://chrome.google.com/webstore/detail/gsconnect/jfnifeihccihocjbfcfhicmmgpjicaec