Remove the Firefox extension building code.

Firefox no longer supports this legacy extension and it is a pain to
sync changes from here and mozilla central.

README.md

@@ -27,15 +27,9 @@ Feel free to stop by #pdfjs on irc.mozilla.org for questions or guidance.
 
 ### Browser Extensions
 
-#### Firefox (and Seamonkey)
+#### Firefox
 
-PDF.js is built into version 19+ of Firefox, however, one extension is still available:
-
-+ [Development Version](http://mozilla.github.io/pdf.js/extensions/firefox/pdf.js.xpi) - This extension is mainly intended for developers/testers, and it is updated every time new code is merged into the PDF.js codebase. It should be quite stable but might break from time to time.
-
-  + Please note that the extension is *not* guaranteed to be compatible with Firefox versions that are *older* than the current Nightly version, see the [Release Calendar](https://wiki.mozilla.org/RapidRelease/Calendar#Past_branch_dates).
-
-  + The extension *may* also work in Seamonkey, provided that it is based on a Firefox version as above (see [Which version of Firefox does SeaMonkey 2.x correspond with?](https://wiki.mozilla.org/SeaMonkey/FAQ#General)), but we do *not* guarantee compatibility.
+PDF.js is built into version 19+ of Firefox
 
 #### Chrome