Relative paths for Javascript fetch inside a Thunderbird extension

Question

I'm trying to use window.fetch() in a Thunderbird extension I'm working on. Specifically, I want to try fetching the contents of files using relative paths - but am failing, possibly because I'm not using the right relative path.

So, the scenario is that the extension provides a background.html; that include a script file, background.js; that script invokes the "WebListener API", an extension loader of sorts; the WL API code registers a JS file's URI I give it in a property of a window object; then, finally, when that window loads, Services.scriptloader.loadSubScript() is called with my JS file's URI; and this invokes my actual code, where I call window.fetch(). Yes, I realize that is rather contorted and weird, but that's just the way it is these days :-(

Anyway, the code I'm calling is something like the following:

window.fetch(uri) .then((response) => { if (!response.ok) { throw new Error(`thrown response status: ${response.status}`); } return response.text(); }) .then((text) => doStuff(text) ) .catch((err) => console.error(`fetch failed: ${err}`)); 

This works (mostly-works, anyway) if I specify URIs such as chrome://bidimailui/content/subdir/somefile.xhtml - and bundle somefile.xhtml in the appropriate location within the XPI. But if I try specifying a relative path - relative to the location of background.html, or background.js, or the script where the code snippt above is located, or with some parts of the path prefixed - none of those options work. I get an opaque "Network error", in the .catch() block.

My question: Is this relative-path fetching even supposed to succeed? And if it is - how do I determine the correct relative path for the file I'm trying to fetch? i.e. what would it be relative to?