every from rule should contain a slash after the host part
A from rule that matches on an entire site, but doesn't contain a trailing slash, could be misinterpreted.
Experiment seems to show that an explicit trailing slash on the from rule for a site does not cause Firefox to fail to apply the rule if the user doesn't type the slash. For instance, the from rule ^http://www.example.com/ will correctly trigger if the user types "www.example.com", "http://www.example.com", "www.example.com/", or "http://www.example.com/" in the address bar. Adding the trailing slash avoids potential false positives and does not seem to create any false negatives, so it should be done by default in rules that ship with HTTPS Everywhere.
This does NOT mean that ^http://www.example.com/resource and ^http://www.example.com/resource/ are the same. The trailing slash is only automatically appropriate at the top level of a site, not necessarily for individual pages of the site. Whether trailing slashes belong in rules referring to individual pages or directories is a case-by-case question.