Government employees have free speech rights when discussing a matter of public concern. Except when it disrupts the workplace. Or when the speech is made pursuant to their job duties. Citizen speech is protected. Official speech is not.
It's a convoluted rule, this standard borne from Garcetti v. Ceballos. How does one determine if speech is made as a citizen or as an employee? As the Wait a Second! blog notes, Garcetti was initially interpreted narrowly (speech is protected unless that speech is mandated by one's job duties) but has since been broadened by the circuit courts to include pretty much anything that you learn about in the line of duty.