Firefox 73 SSB on Unity often results in a blank white window and also is lacking some basic functions
browser.ssb.enabled = true $firefox --ssb https://whatever
Often opens just a blank. Some times works as it should.
Also there's no way to go back/forwards and some other base HTML navigation is broken.
SSB is not a production feature yet, and the code in Firefox 73 is a few months old. For testing, please use either Developer Edition or Nightly. These can install side-by-side with Release.
This explains everything, thanks