Update README.md

README.md

@@ -26,12 +26,15 @@ using the pdf.js API.
 
 ### Extension
 
-A Firefox extension is also available:
+A Firefox extension is availble in two places:
 
-+ http://mozilla.github.com/pdf.js/extensions/firefox/pdf.js.xpi
++ Stable Version: https://addons.mozilla.org/en-US/firefox/addon/pdfjs
++ Development Version: http://mozilla.github.com/pdf.js/extensions/firefox/pdf.js.xpi
 
-Note that this extension is self-updating, and by default Firefox will auto-update extensions on a 
+The development extension should be quite stable but still might break from time to time.
-daily basis (you can change this through the `extensions.update.interval` option in `about:config`).
+Also, note that the development extension is updated on every merge and by default Firefox will
+auto-update extensions on a daily basis (you can change this through the 
+`extensions.update.interval` option in `about:config`).
 
 For an experimental Chrome extension, get the code as explained below and issue `make extension`. 
 Then open Chrome with the flag `--enable-experimental-extension-apis`, go to `Tools > Extension`