OSHA says that you don't have to perform a job function if you don't feel that the conditions are safe. Refuse to open boxes without a sharp implement, because with a dull one, or your fingers, you could wind up injuring yourself on hidden staples, paper cuts, etc.
Before you do anything, call your state board of labor/unemployment office, and find out their views on unsafe working conditions, and your remedies if you are fired for refusing to follow an unsafe instruction. They usually have pamphlets with the guidelines. Get one, highlight the relevant passages, and drop it on the floor in front of your employer as you stroll out the door. Make sure he knows that your fellow employees are also aware of the guidelines.
When he fires you, have the address of your local unemployment office handy. The benefit won't be much less money than you're making now, and it will, if he keeps ticking off employeesm, ultimately cause his unemployment insurance premiums to increase. Also, file a complaint with OSHA and the state labor board. You will probably be able to recover any wages you lose, and your ex-employer may be hit with a nice little fine. In these cases, the state/feds are your representatives - the employer can either show up alone or with an attorney that he has to pay for. In any case, the employer always loses these things.
It sounds like you work for a real lame-o. The only way you're going to be able to change your situation is to find work elsewhere. But it's always nice to leave the loser with a spring in your step knowing that the state and the feds will be paying him a call to remind him of you.