I googled for an answer prior with no result. Posting my finding.
Lightbox is running an HTML5 player (beta?/trial?) atm.
My platform mint cinnamon on 5 y/o core duo machine:
Google Chrome Browser (yes) - working well enough.
Chromium Browser (no) - video loading progress indicator but nothing else.
Firefox Browser (no) - says Linux is not supported etc.
I have not investigated the issue with Chromium (maybe flags) or Firefox (maybe force HTML5 player).
Running closed source and Google on otherwise open source platform is not a happy combo so I hope someone will find solutions for these browsers ; ).

