Temporarily turn off mixed content blocking, with prompt
Firefox 23's release date is August 6, and it comes with a mixed content blocker. Unfortunately this will break a lot of websites for HTTPS Everywhere users, and we need a quick fix mitigation until the real bugs around this issue can be fixed.
The quick fix is this. If Firefox is >= 23 and MCB is on, on HTTPS Everywhere upgrade prompt the user to turn it off temporarily.
There are two components to this: the logic and the UI. There needs to be new about:config preferences, at least a flag for whether or not this prompt has been shown. For now the UI can be quick and sloppy. We'll work with EFF's graphics design to figure out the least intrusive way to display this prompt, along with all the other prompts, and not overwhelm the user.
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