wss connection interrupted error in firefox, works in chrome

I'm using the latest paho-mqtt.js from master and i'm testing it against the server at test.mosquitto.org on port 8081. In chrome, everything works fine, but in firefox I get:

The connection to wss://test.mosquitto.org:8081/mqtt was interrupted while the page was loading.

I've tried putting the connection kickoff at window onload instead of via a connect button, but got the same error in firefox.

Curiously, if I change to an http server instead of https and change the code to not use "useSSL" in the connect options (along with switching to port 8080), then everything works fine in firefox. It is only when https / wss is being used that I keep getting this error and can't get any connect / subscription to work in firefox.

This is using Firefox 78.4.1esr (32-bit)