Will My Positive Employee Appraisals Protect Me If They Try To Fire Me?
The answer is almost certainly NO.
First read the post on understanding the labor laws and disclaimer here (will open a new window).
While you may believe it's unfair that companies can fire anyone, regardless of how well they perform on the job, they can do so, provided they don't stumble on OTHER laws having to do with discrimination on the basis of your "protected class" membership (i.e. race, gender, disability).
While laws vary, companies can fire their best performers if they want. In fact, sometimes companies will let go the highest paid employees, who may also be the best performers - go figure. It may not be good business, but it's their businesses.
Obviously, it's "better" if you are let go, to have positive reviews, since they can be helpful in getting a new job, or helping your former company write a good letter of reference or endorsement.
Or, if you want to question a manager's decision, by going to human resources, at least you have some ammunition to make a case, if the manager is claiming poor performance.
Almost No Legal Protection Even If You Have Positive Reviews
Ultimately, positive performance reviews will not offer you much if any legal protection if your company decides to fire you or lay you off. Please read the two limited exceptions here to determine if they fit for you.