Promote FreshRSS-Notify in user documentation (#2445)

* Add FreshRSS-Notify in Clients compatibles section

* Add FreshRSS-Notify in Compatible clients section

* Use language-neutral link

https://addons.mozilla.org/firefox/addon/freshrss-notify-webextension/

docs/en/users/06_Mobile_access.md

@@ -55,6 +55,8 @@ See the [page about the Fever compatible API](06_Fever_API.md) for another possi
 		* [FeedReader 2.0+](https://jangernert.github.io/FeedReader/) (Open source)
 	* MacOS
 		* [Vienna RSS](http://www.vienna-rss.com/) (Open source)
+	* Firefox
+		* [FreshRSS-Notify](https://addons.mozilla.org/firefox/addon/freshrss-notify-webextension/) (Open source)
 
 
 # Google Reader compatible API

docs/fr/users/06_Mobile_access.md

@@ -69,6 +69,8 @@ Tout client supportant une API de type Google Reader. Sélection :
 	* [FeedReader 2.0+](https://jangernert.github.io/FeedReader/) (Libre)
 * MacOS
 	* [Vienna RSS](http://www.vienna-rss.com/) (Libre)
+* Firefox
+	* [FreshRSS-Notify](https://addons.mozilla.org/fr/firefox/addon/freshrss-notify-webextension/) (Libre)
 
 # API compatible Google Reader