"Employers can legally make it a condition of employment that you not own any firearms."
Name the federal, state, or local codicil that allows this. You are saying 'legally' so tell me where this is legal that an employer can dictate what you do or do not own on your own private property.
So here's your argument about employer's "rights" being extended onto YOUR private property - where you have previously maintained that your private property rights trump any other consideration.
In contract law, you can agree to anything that is not otherwise forbidden under local, state or federal law. You can agree, as a condition of your employment, not to own firearms. Of course, if such a rule is too onerous, you can always quit. If you can show me a law that would forbid this type of agreement in a given jurisdiction, that's a different matter. Absent such law, however, such a rule by an employer would be perfectly legal.
So here's your argument about employer's "rights" being extended onto YOUR private property
You are only bound by this agreement because you have consented to it as a condition of your employment. Again, if this rule is too onerous you are free to quit and seek employment elsewhere.