I’ve noticed that cloudflare seems to let certain request through if you’ve done a similar request through the browser already - but only if it’s from the same device. I think there’s some kind of whitelist period after you hit it with a browser, which makes this incredibly annoying to test with actually.
Taking the cookie from a browser and sending along with the calibre plugin request doesn’t seem to work either
Re: Firefox - it wouldn’t be able to fill in the calibre metadata. If all you wanted was the cover, it’s not too difficult to grab it manually from the page with the browser.