Could household breathing masks protect against secondhand smoke?

Not much: it might screen out some of the larger particulate matter, but not the smaller particles as well as the molecules in the smoke, like formaldehyde and carbon monoxide. The best treatment is to prevent it. If people are smoking in your home, don't allow it. If you don't have a choice, try negotiating a 'smoking room' which could be outfitted with "smoke-eaters." Check with your towns laws on this.