Update Firefox version specifications

See travis-ci/travis-build#859

Author: BanzaiMan

user/firefox.md

@@ -4,7 +4,7 @@ layout: en
 permalink: /user/firefox/
 ---
 
-Our 64-bit Linux VMs include a recent version of Firefox, currently 31.0esr.
+Our 64-bit Linux VMs include a version of Firefox.
 
 While Firefox is not pre-installed on OS X images, you can use this addon to set it up for use
 on your builds.
@@ -13,10 +13,13 @@ on your builds.
 
 To install a specific version of Firefox, you can use the Firefox addon. The addon will download and install Firefox before running your build script.
 
-For example, to install version 39.0 of Firefox, add the following at the top level of your `.travis.yml` file:
+For example, to install version 49.0 of Firefox, add the following at the top level of your `.travis.yml` file:
 
     addons:
-      firefox: "39.0"
+      firefox: "49.0"
+
+
+It is also possible to specify beta versions; e.g., `50.0b6`.
 
 ## Version aliases