Can I seal a fire wall with expanding foam?

Not with ordinary foam. It burns.

Builder's expanding foam and general silicone are not tested as fire seals. In a fire they give way and leave the gap open.

Fire-rated foam does exist. It is a different product, tested for the job, and usually pink or red. You can see some round the steel tray in the photo.

Colour is a hint, not proof. What counts is a product with a test for the wall, the gap and the service passing through it, fitted the way the test says.

If you have foam in a fire wall and no label to say what it is, treat it as unproven. A photo of it sent to us is enough for a first opinion.

For what to do next, see fire stopping in Maidenhead.