Firefox search box typeahead completion leaks plaintext queries
Even if an HTTPS Everywhere rule matches the relevant URL, typing text in the Firefox search box will send unencrypted HTTP queries to the host defined for typeahead completion in the search engine definition file.
HTTPS Everywhere thinks it is rewriting these queries (according to the error console), but a packet sniffer verifies that the rewriting never occurs and the queries are actually sent as plaintext! (To be more precise, the protocol scheme and host are never effectively changed; a rewrite rule can still have an effect on the path part of the URL.)
See https://mail1.eff.org/pipermail/https-everywhere/2010-July/000025.html for more details.