Was I unlawfully terminated?

I was fired a few months ago for making a post on Facebook complaining about my salary. Specifically, I included a pic from a hiring sign at my local Target and said that "after 2 college degrees and 3 years at my company" I was making the same hourly rate as a brand new employee at Target.

I don't know how, but my employer saw it and fired me the next day. They waited until I had left for the day then called my cell phone 20 minutes later and said they didn't like the post and I was fired. I pleaded with them to change their mind, but told me that I had a bad attitude (they gave me no specific examples and had never spoken to me about having a bad attitude, prior to being terminated.) I told them I would take the post down right away and pleaded to let me come in and talk face to face, but they said that there was no changing their mind.

Since PA is an at-will state I assumed that they could do whatever they wanted. But a friend said what they did was illegal? Was it?