Move self-executing function to end of tabzilla.js. Bug 950121

Kind of a tricky one to track down and test. From the bug:

> The self-executing function that calls the init function in tabzilla.js is declared before the content var is initialized (possibly) resulting in undefined content.
> 
> This bug has not shown up on mozilla.org likely due to init being called within $(document).ready. mozilla.org's doc.ready must take long enough for the parser to evaluate the content var's initialization.