Bug 838795 - [Browser] URL bar displays "[object Object]" after viewing ...

...default tab tray

Similar to https://bugzilla.mozilla.org/show_bug.cgi?id=838672, the tab.title property may be set to an object or whatever the value of event.detail is, this simply adds a logical guard against accidentally attempting to display an "object" where a string is expected.

https://bugzilla.mozilla.org/show_bug.cgi?id=838795

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>