Fix Bug 935719 - Tabzilla: Create an out of date version warning for users not on the current version

There are no new strings included so this can be merged without l10n.
I'm getting a GA error here:

``The _trackEvent API expects a string value for argument 2. The argument given (25) was a number.``

Possible easy fix would be to convert to a string? ``String(userVersion)``?