i think this should work on fennec too. instead of testing against `getTabURI`, i'm testing against `window.location.href` inside [`_onContent` below](https://github.com/mozilla/addon-sdk/pull/1309/files#diff-401a3bfd1210c470fb30bec33f56ea82R182).

and that `window` reference is acquired in a fennec-friendly way, by calling `[getTabContentWindow](https://github.com/mozilla/addon-sdk/blob/master/lib/sdk/tabs/utils.js#L198)`, which in turn calls `getBrowserForTab`, which [checks for/should work on fennec](https://github.com/mozilla/addon-sdk/blob/master/lib/sdk/tabs/utils.js#L170).