Fix Bug 935719 - Tabzilla: Create an out of date version warning for users not on the current version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.
Need a bit of cleanup here. Since we're ignoring the fact that we have ESR users for this feature (until the UITour is in Release), this `if` should just be:

```if (userVersion < parseInt(latestVersion, 10) - 2) {```

Should update the comments above as well to make it clear we are *not* filtering out ESR users.