This is the error I'm getting from your client:
Client error reported: nos2x-fox: invalid base64: Unknown letter: "=". Allowed: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/ decodePayload@moz-extension://d1971818-737b-4639-b82d-2dd5359423e1/background.js:7643:13
decrypt22@moz-extension://d1971818-737b-4639-b82d-2dd5359423e1/background.js:7665:53
handleContentScriptMessage@moz-extension://d1971818-737b-4639-b82d-2dd5359423e1/background.js:8328:35
You might have an old version of Nos2x. I tried using that extension and it worked fine for me.
@fiatjaf is this something you updated?