Even if it weren’t government-subsidized, there is still wrongful firing. *IF* ... and it is not clear if this is the case ... the employees reasonably responded to a request to remove the scripture verse, or if the scripture verse were not displayed, the firing may still have been illegal.
But I think I do agree with Mariner’s point: If Hallmark doesn’t want their business associated with Christianity, they do have the right to present their business to the public as they see fit.
There's no such thing as a wrongful firing, unless it violates the provisions of an employment contract.
The power of boycott is a very effective tool for sending messages to companies which are overtly hostile to religion.