How to detect Android's version number in Firefox for Android?

Question

For a website we want to show a link in the play store if the user comes with an Android 4+ device. All browsers we tested sent the version number of Android in their user-agent

Answer 1

Unfortunately, you cannot. See Bug 625238 - Add device info to User-Agent and the related Bug 807495 - User Agent does not include handset Device Model.

In general, Mozilla has a policy of not adding User Agent (or HTTP header) information, and they currently have the opinion that developers do not need to know the running Android version.