As Lucas pointed out in bug 997477 comment 28, this method is no longer used. However, I tried simply removing it (and moving the forward button animation code to the stopEditing method) and the back/forward button state could become inconsistent when switching between tabs or entering editing mode. I imagine this is because we change the button state via onTabChanged calls, and we would just override this state when exiting editing mode anyway (or similar).
mcomella, maybe this is something we'll be able to get rid of in the new tablet UI?
I wonder if we won't see an issue like bug 1001243, but I'll give it a go.
Actually, this is currently only a method of BrowserToolbarTablet (and not BTNewTablet) but this may be because I have not created an "editing mode" yet. I'll probably end of copy-pasta'ing the editing mode code over when I implement bug 1058902, but I'll do my best not to re-add this method.
This shouldn't be re-added on new tablet, at least not in same form, because we don't need to disable button state.