I've tried a new profile and no extensions and still see the
weirdness with the back button.
It may well be gtk as the button normally has a bit of
a fancy GUI effect when it is clicked, and no such
effect shows up when it is ignoring it. When it finally
does react to the button, I see both the button
effect and the action of actually going back to
previous page.
For grins, I tried installing google chrome, and it has
no such problems (it merely constantly prompts
me to give my soul to google :-).