I am not signing a non compete agreement after having worked here 7 years. Can my boss fire me for no signing?

Probably.  In most states (I don't know if Montana is different), unless you have a contract which limits your employer's right to terminate you (i.e. a contract which provides for a specific duration of your employment, or one that provides that you can only be fired for good cause), you are probably what is known as an "employee at will", which means that you can be terminated at any time for virtually any reason that is not specifically prohibited by a statute (i.e. being fired because of your religion or race or because you reported serious safety violations by your employer, etc.)  Sometimes a contract can be created from an employee manual which provides, for example, that no one will be fired without good cause.  Depending on the particular circumstances of your employment, this contract may not have to be in writing.  It could be a collective bargaining agreement between your employer and your union.  However, most employees are employed at will, and if you are an employee at will you can be fired for refusing to sign a non-compete agreement (known as a restrictive covenant).