I live in an old house -- can I squirt aerosol insulation into area behind faceplate before I attach new faceplates?

You are better off installing pre-cut foam. The reason is that aerosol insulation tends to contract and pull away from the studs over time, allowing infiltration again.

pdworkin said it all. in addition, i would be sure to read the label to make sure the insulation does not contain a harmful ingredient that will effect your breathing. some of the insulation ingredients are harmful to humans and they should not even be on the market.

You should replace the plugs and the faceplates. I would not recommend spraying that insulation around the wires. You can insulate the face plates around the inside edge with
insulating tape. Is it an outside wall?

If you ever need to have an electrician go inside that outlet box for any reason he’s going to be plenty pissed at you for encapsulating everything inside. That old knob and tube wiring is brittle by this time so you won’t get many chances to bend the conductors before they snap. If they break off too short to where they cant be reconnected you will have pretty much screwed the pooch. A new wire will then need to be snaked up to connect at the next box where that set of wires might break when the splice is made,etc, etc, etc. Lots of wall cutting and patching to do this usually so it could turn into an expensive disaster. Use the precut foam wafers for the switches and outlets you will be glad you did.