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Did you actually uninstall the old snap version? It feels like it's still being boxed in.
Unfortunately, when I hit situations like this, I usually ended up using Chrome or some other browser for whatever service I needed. For some reason Chrome wasn't distributed in a snap. The problem with Chrome was that eventually the packager for Alphabet stopped updating the distribution, even if you were using the long term kit.
If you open up the .json file with a file editor there will probably be a path attribute with a file path to the executable you mentioned earlier. You can check that they match.
My ~/.mozilla/native-messaging-hosts actually has two files in it (one for keepass, and another for a tiddlywiki adjunct.) Both of them have path attributes.
You might also check that the executable isn't 0 bytes and that it has appropriate rights and ownership.
The extension should talk to the native host, either when you navigate to a web site that knows how to use it, or when you click on the extension (if you've pinned it).
The JSON file is probably in the correct place. It probably is the manifest.json?
There's a value inside the manifest.json that should match the id of the extension in the extension details for your extension.
I'm assuming that you already have an application on your machine that uses Sconnect?
Those instructions are for installing the native host. It's the executable that the browser extension knows how to connect to.
On the page I posted, there's an xpi. I would expect that you download that and side-load it via the FF extensions page. Might require developer mode for that.
That's the way similar projects that I've used work. A side-loaded xpi that connects to a local executable.
The arrow might point to your bookmarks. So maybe check if you have new bookmarks?
Did you get to this page? They seem to have an xpi for you to sideload.