Is expanding foam fire stopping?

Not the foam in a can from a builders merchant. That is insulation, and it burns.

Fire rated foam does exist. It is a different product, often pink or red, and it has been tested for particular uses.

But even that is not "fire stopping" on its own. A fire stop is a whole system: the foam, the size of the hole, what goes through it, what the wall is made of and how long it has to hold.

Change any of those, and the test no longer applies.

So when someone says they filled the hole with foam, ask which foam, which tested system it belongs to and for what rating.

A label on the seal should answer all three.

Gaps that have only plain foam in them are common on surveys. They count as unsealed, and they get cut out and redone.