Is expanding foam fire stopping?

Usually no.

The ordinary foam in a can from a builders' merchant burns. It is meant to stop draughts and fill gaps for insulation. It is not made to hold back a fire.

Some foams are made for fire stopping. They have been tested in a wall or floor of a particular kind, with a particular service going through, and they come with a data sheet that says so. These are sold for the job and cost more.

The two can look much alike. Fire-rated foam is often a different colour, but you cannot rely on colour. A tested product has a label or a data sheet, and a tested system has a stated opening size and depth.

So how do you tell what is in a hole? Look for a label or a sticker nearby. Look for a record from the installer. If there is neither, assume nothing and ask for a survey.

If you find ordinary foam in a fire wall, it is likely to need replacing. Adding more on top does not fix it.