I understand your situation. I worked for many years in unemployment and heard many terrible stories from people who worked for incredible jerks. It is hard to realize that you can, really, legally, be treated this way and there is no labor law or fairness law that prohibits an employer from having a business and treating his employees as bad as he can get away with, which means that he can do it, and if he can find employees who stand there and take it, it continues. The good ones will realize what sort of jerk and bully he is, and they'll leave and get a better job and leave him behind. Your state not only is an "at will" state in that he can fire you at will. It is also a state in which you can quit any time, without giving him notice, without any restriction.
However, my word to you is that since you have put up with this behavior for so long, DO NOT QUIT this job until you have another one with a firm start date. Then just walk away and leave him behind. People like this are probably never going to give you a good reference anyhow. While you are looking for other employment, don't spend a lot of time trashing him, either, but you may wish to mention that you'd prefer the prospective company not let them know you're seeking work elsewhere.
And if he fires you this Monday, which I'm betting he will not do, or if he find out you are looking for another job and fires you, you do have one tiny bit of recourse. It is not 100% guaranteed that if you do quit, you won't be approved for unemployment benefits while you are looking for another job and if you are fired it is 100% sure you will be approved for unemployment, but it is much more likely to go this way.
If you quit, you must have a valid work related reason to quit the job. If he slaps you, or dramatically changes your days hours or job assignment, walk out and quit (and if he physically assaults you, call the police and press charges while you are walking out!) right then, but unless it is a really big clear cut reason, you probably will not get approved for unemployment if you quit. Standing six years of many rotten experiences and insults and such and then quitting is probably not going to be seen as a valid reason to quit.
But if he fires you, he must show that he has a clear cut misconduct reason to terminate your employment. That he has let you know that he was dissatisfied with your work, that you have received warnings, and have refused to change your behavior and so were terminated, or you did something really bad, like steal money from the company, which would be gross misconduct. Otherwise, if he acts on one of his bad mood whims and fires you summarily without a good reason, you are much more likely to be approved for unemployment while looking for that other job.
And that is the reason there is an unemployment insurance program. It is so that jerks cannot completely control your life and your income and fire you on whim without any repercussions to them. Paying unemployment benefits costs them money, they usually hate to do it. And they do not get to approve or deny you benefits, it is a decision made by the unemployment system.
But you work and begin a diligent work search for a better job where you will be treated better. No one can fix a bad workplace, all you can do is move on. When you have that new job and are ready to start, move on. And all you can do for your co-workers is set a good example of how a professional mature good person deals with working for a jerk. You move on and do better. Good luck to you.