Mozilla is asking all Firefox users to upgrade immediately to version 39.0.3. Anyone on the Firefox Extended Support release via their school or business should upgrade to version 38.1.1.
The security issue only affects PCs since the flaw relies on an interaction between Firefox’s PDF Viewer and other parts of the browser. Firefox for Android does not have the PDF Viewer and therefore not vulnerable, according to a blog post by Mozilla’s security lead, Daniel Veditz.
Mine updated automatically. It should automatic for most people.