The thing that jumps out at me is the way you asked your question. Mainly the summary. I'm guessing something you may not have considered.
In your summary, you began by saying , "We've" been debating firing an employee. Well, that employee is obviously your subordinate if you have the authority to fire them. The term "We've" tells me that your husband is an intricate part of your company's decision making. A partner, if you will. So, he takes the place of the fired employee. Have you decided if he will be a partner or a subordinate and be able to carry out his assigned responsibilities without questioning your decisions like any other employee would be required to do? Or, are you ready to give up some of your authority and make him an equal? Sounds like this is a company that you created.
The other thing is, what will be the reactions of your other employees , if you have any? Will they look at him as an equal or a boss? This could be difficult for them , and him also, but you can't have both.
Please realize, if he works with you, in any capacity, it will effect both of your work lives and your home life. If you disagree about something at work, you'll carry that home. Disagree at home, carry that to work.
If your business runs into rough times, who will carry who? Are you putting all your eggs in one basket.?
Remember, if you work together, your work life and marital life will have a tremendous impact on each other. Maybe, hiring an outside person would be much less stressful to your lives as a whole.
Good Luck in whatever you decide. I appreciate that you are open-minded enough to even ask for outside opinions. Owners seem to have a tendency to be arrogant and have big egos. You've shown neither.