This issue only affects Firefox for Android. Other operating systems are unaffected.
The Mozilla Foundation Security Advisory describes this flaw as:
A malicious Android application could craft an Intent that would have been processed by Firefox for Android and potentially result in a file overwrite in the user's profile directory. One exploitation vector for this would be to supply a user.js file providing arbitrary malicious preference values. Control of arbitrary preferences can lead to sufficient compromise such that it is generally equivalent to arbitrary code execution.