Sure they can fire you. They don't even need a reason. But, if they fire you for having a gun in your vehicle, you can sue and recover damages and attorney's fees. That is what the provision says. Also, they cannot search your car because they want to. They can force you off the property, but they cannot search without your consent.
(4) An employer that fires, disciplines, demotes, or otherwise punishes an employee who is lawfully exercising a right guaranteed by this section and who is engaging in conduct in compliance with this statute shall be liable in civil damages. An employee may seek and the court shall grant an injunction against an employer who is violating the provisions of this section when it is found that the employee is in compliance with the provisions of this section.