Bug 1080634 - Update the Install link to point to a more relevant help p...

...age + Provide different URLs for Desktop & Mobile
FYI, this logic is used in the file  https://github.com/mozilla/bedrock/blob/master/bedrock/firefox/templates/firefox/releases/release-notes.html#L33
Firefox OS does not have this part. https://www.mozilla.org/en-US/firefox/os/notes/1.4/
ESR is already pointing to the same old url https://www.mozilla.org/en-US/firefox/31.1.0/releasenotes/ 
Thunderbird is not handled on bedrock