That's mysterious, as it doesn't work on Firefox at for me. In Chrome after it says "READY" I can use the keyboard below it to things.
On Firefox I get the "READY" but am then unable to use the keyboard or the drop-downs. When I click on a drop-down and wait for half a minute it might finally show it, but it's unusable. Firefox also regularly comes up with a warning that the page is slowing down Firefox, and the browser indeed seems to be hogging a single core.
This is in Firefox 126.0 on Fedora 39.
[update: I tried with safe-mode to disable any extensions but this doesn't make a difference]
(This is hard to overcome: the emulator has to render in a 60Hz duty cycle and we're emulating a CPU at 1 Mhz. If sound is enabled, we also have to sample sound at 1 MHz, and resample this to 48KHz digital audio.)
There seems to be some serious misconfiguration at play, though. For me, this runs on the oldest modern machine, I have around, a 2008 MacPro with the original ATI 256MB graphics card, under Firefox 78.15.0esr quite perfectly. This is probably puny in comparison to your setup.
I guess, inadequate hardware acceleration may cause similar symptoms. I've seen this in the past (and in different context) with Chrome on older machines, where the frame rate drops to something like 2 or 4 fps, while it keeps up perfectly with hardware acceleration disabled. May be worth a try, maybe FF and your GPU driver do not play well together. Compare: https://support.mozilla.org/en-US/kb/performance-settings