If i'm sued for slander, can I counterue?

I was sent a letter because my friend started a FB group because I was fired from work. She stated her opinions on the matter of me being fired and how she thought it was unfair and that the company was a joke for doing so. They said I was being sued for slander, yet I haven't said anything mean about them nor have I said anything about being fired on the group's webpage. They said in the letter that I posted something on their page that was slanderous. I have not contacted them in any way after being fired. Can I countersue against them?