Make "Not Now" the default button for TorBrowser's canvas permission dialogue
When TorBrowser's HTML5 canvas permission dialogue pops up from the URL bar,
I suspect that users have no idea what any of that text means, and so they click the largest, most-visible button available on the dialogue box. Right now, that's the "Allow in the Future" button, which allows the site to access HTML5 canvases forever (and until legacy/trac#12682 (moved) and/or legacy/trac#12683 (moved) are fixed, there isn't a way to revoke that permission).
I suggest that we make the largest, most-visible button on this dialogue be the one which doesn't allow the site permission to access HTML5 canvases, i.e. the "Not Now" button, since a site which is trying to do this is overwhelmingly likely to be sourcing some evil ad company's scripts which try to track users. It's not fair to make normal users understand how this works, so let's use people's desires to make popups go away ASAP to their own advantage for their privacy, without making them think too hard about it.
For background info, see this thread on the tor-talk mailing list.