Throughput seems to be unaffected though, so the effect is primarily delayed, but not slowed down, rendering. That delay still could still cause queues to build up and confuse the congestion control.
We haven't found a workaround yet. It doesn't seem to matter which method we use to wait for the image to load.
There is still some delay, but it's similar to Chrome now. The performance of running an entire noVNC playback test is also better on Firefox than Chrome now. Not sure if we need to care about this anymore.
(Firefox can get even faster loading using newer APIs: https://github.com/novnc/noVNC/pull/1131)