Earlier quoted context omitted.
Gonna have a hell of a time proving the boss's tweets have anything to do with working conditions.
I don't think that's the burden of proof though, I think they have to prove they were fired in retaliation.
You can fire people in retaliation for stealing or screwing up or for wasting everyone's time complaining about Twitter drama instead of doing their job or lots of other reasons. Simply firing in retaliation is not illegal.