One thing you might want to look at is embedded WebExtensions: this lets you embed a WebExtension inside a traditional bootstrapped extension, so you can gradually port it a piece at a time. However, I think you can only do this with a bootstrapped (i.e. restartless) extension or an SDK add-on. So you can’t do it with a restart-required, XUL overlay-based extension.
You might want to make the URL look cleaner in the tab though, that’s what I like. So I would register an about page with URI_SAFE_FOR_UNTRUSTED_CONTENT which makes your html page have no XPCOM access.