HTTPS Everywhere no longer works in SeaMonkey

Ever since the conversion to WebExtensions, HTTPS Everywhere has not functioned in SeaMonkey. SeaMonkey 2.49.3 (the current version) is based on Firefox ESR 52.7.3, but doesn't incorporate what 52.x has of the WebExtensions API.

In this blog post, it is stated that WebExtensions support is not present. According to IRC conversations with devs, that hasn't changed in the past year.

I can see four options here.

1. This is the easiest one. Just drop SeaMonkey support entirely. Remove the block/line from the manifest (or whatever it is now) that says "I support SeaMonkey, too!"
2. Make a tool that converts from the new ruleset format to the old one so that SeaMonkey users can update the rulesets themselves.
3. Update the XUL version not to match the new one in code and features, but simply to parse the new ruleset format. Maybe add a conversion method when it initially reads the rulesets in so that nothing else needs to be changed.
4. Use the tool mentioned in option 2 to make new XUL releases that only update the rulesets.

Trac:
Username: NiveusLuna