employment in the US is, practically always, classified as "at will", which means either the employee or the employer can end the employment at any time for (almost) any reason. there are very narrow exceptions to this. missouri states that employees cannot be fired solely for their political beliefs. however, posting offensive jokes that bother your coworkers will be difficult to defend as just a "political belief". the case itself gives an example that is extremely coarse. it'll be an uphill battle for sure.

"Law" (whatever that means to you) does not, in the main, protect you from the private consequences of your speech.

It turns out that when you say shitty things about your coworkers in ways themselves violative of the law, people have no obligation to keep you employed!

This is a lawsuit decrying the reality of finding-out post fuck-around.