For a while now, spellcheck has been disabled on Firefox due to not supporting certain browser APIs that we rely on. New versions of Firefox have the option to turn on those APIs and soon it will be enabled by default.
To enable spellcheck on Firefox:
Download Firefox version 81
Navigate to about:config
Search dom.input_events.beforeinput.enabled and turn it on.
Once you turn it on, refresh your docs. It should start working automatically!