Contractors for Google are still employees working for a contracting company. They could sue if they were discriminated against for membership in a protected class.
But even if they were employees, they are not really getting fired from the contracting company - it can still try and find them another gig, for a different client. So you can say they are not getting fired at all, just suddenly have no work to do (but still have a valid employment contract).
Of course if contracting agency can't find them another position in reasonably short time they will fire them as well, but that's not on Google anymore, so Google can't be sued for that.