"Employed in FL but reside in Mass"
You are not employed in Florida. You are employed in Massachusetts by a company that has its headquarters in Florida. You work in Massachusetts, not in Florida.
"Is my FL employer subject to pay Massachusetts tax simply because that is where I reside?"
The wording of your question sounds like you are asking about the implications for your employer, not for yourself. Are you asking (a) whether you have to pay Massachusetts tax, (b) whether your employer has to withhold Massachusetts tax from your pay, or (c) whether the company that you work for has to pay Massachusetts business tax on its income?
Under Massachusetts law, if the company you work for is a corporation, having even one employee in Massachusetts (such as you) means that the company is considered to have a presence in Massachusetts and is therefore "subject to the tax jurisdiction of Massachusetts." A common term for this is that the company has nexus in Massachusetts. It's not because you live in Massachusetts; it's because you work in Massachusetts. Having nexus means that the company has to file a Massachusetts corporate tax return. It probably also means that they have to withhold Massachusetts tax from your pay.
I am not a lawyer or an expert on Massachusetts taxes. This is just what I've learned from a quick bit of research. Your employer should consult its own lawyers or tax advisers concerning the implications of having an employee in Massachusetts.
