[Firefox] Use getTabForBrowser instead of _getTabForBrowser if possible

`_getTabForBrowser` was depreciated in https://bugzilla.mozilla.org/show_bug.cgi?id=1039500, so this patch adds support for `getTabForBrowser` in a way that doesn't break backwards compatibility. (This is in contrast to [this already landed patch](https://bugzilla.mozilla.org/attachment.cgi?id=8502864&action=diff), which if up-streamed would break the findbar in older versions of Firefox.)