In Firefox (v82) the text-field does not seem to render in a strictly non-proportional font. Thus everything is garbled. Unfortunately w3m text-mode browser isn't capable to display the page...
[EDIT] The following steps fix playback in Firefox for me:
- I press F12 to open the web-developer tools,
- select the "Console" Tab
- at the prompt at the bottom (marked by ">>") enter:
document.querySelector("pre#screen").setAttribute("style", "font-family:Monospace;")