Hi. I'm just a user not an addon dev but I think I migrated HTTPS everywhere to WebExtensions. It is signed and you can download it from here.

This is how I did this. I installed this addon went to chrome web store and installed in inside Firefox.

I downloaded the xpi from Chrome web store with the Foxified extension, unzipped it and I ran this commands:

npm install -g web-ext
web-ext sign --api-key $API_KEY --api-secret $API_SECRET -s HTTPS_Everywhere

HTTPS_Everywhere is the folder with the unzipped xpi addon.

Why migrate to web extensions? They use async APIs so they are faster, only one codebase for both browsers.