Rated 5 out of 5 stars
BC signature capability is still unmatched!
Although Firefox reports that this add-on will not work beyond version 6, here is a method to continue enjoying:
In the address bar of gmail, delete the "s" in "https" then hit enter. The page will reload and you will notice that the the address still shows "https". And now you will notice that Blank Canvas also works.
From my observations in Chrome, I believe that what is happening is that, although the connection to gmail is still secure, some elements such as needed by BC are allowed to be transmitted unsecure.