Breaks goodreads.com / cloudfront.net
Visiting http://www.goodreads.com/ with HTTPS Everywhere enabled causes some functionality of Goodreads to be broken. In particular, some of the images are not properly displayed.
To test: Create an account on Goodreads. Go to the web page for some book, say this one: http://www.goodreads.com/book/show/29579.Foundation Rate the book 4 stars by clicking on the 4th star. Notice how the leftmost 4 stars are now lit up to orange. Now reload the page. You will find that the orange is gone; all five stars are now shaded grey.
Possibly related: If you open up the Firefox Error Console, you will see lots of errors. Here is one that seems to be representative:
Error: [Exception... "'Image HTTP->HTTPS redirection to https://dkt27ch3b0vq7.cloudfront.net/images/loading-trans.gif?1322099299' when calling method: [nsIContentPolicy::shouldLoad]" nsresult: "0x8057001e (NS_ERROR_XPC_JS_THREW_STRING)" location: "" data: no]
There are dozens and dozens of these exceptions in the Error Console.
It looks like Goodreads is using Amazon's Cloudfront service as a CDN, to host images. Thus, the Goodreads web page has a HTTP link to something on cloudfront.net. I'm guessing HTTPS Everywhere is rewriting the cloudfront.net URL to use HTTPS. At some point in this chain it appears that something goes wrong, though I'm not sure what exactly.
I can confirm that if I go to the HTTPS Everywhere preferences, select the Cloudfront site, and disable HTTPS Everywhere for that site, then everything seems to work fine: the page displays properly (the stars on the page show up orange, as they should), and there are no errors in the Firefox Error Console.