I am an employer in Florida in a company that has less than 10 employees. An employee that has returned from maternity leave 4 days ago, is causing problems with other employees, and the company has also lost a lot of money because of her negligence, she said that if we fired her she is going to sue me for a FMLA violation. Does this person have any ground to stand on? Should I be worried or not?

Thanks in advance for any information I receive

As an employer with less than 10 employees, you are not subject to the provisions of the FMLA, anyway.

Nope. She can try to sue you but she won't get very far at all as you aren't subject to FMLA and are too small for the PDA to apply. Technically, you could discriminate against her on that basis and she couldn't sue you (not suggesting you try this by the way, just making a point).