Lonny, if you have an orange theme the icon may be invisible because it's orange.

You can also activate it with control-shift-u (cmd on mac). If you find a bug report it on the issues page in the description.

Edit: sorry I didn't set the version field correctly for the latest update. The addon now uses the new API but this only works on Firefox 57+. The version requirement should be fixed now so you can download the older version of the addon if you're not on Firefox 57 yet.