Bug 838672 [Browser] Default tab title "[object Object]"

Ensure that the value being used to set the display text of a tab is a string (certain event handler logic appear to set the property value to an object).

Remove unnecessary text node creation, assign display text directly to innerHTML of container span node.

https://bugzilla.mozilla.org/show_bug.cgi?id=838672

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>