The visa itself is not transferable to any other employer. the petition and the visa do not grant you general permission to work in the US. You only have permission to work for the employer who filled out your petition. If you want to work for someone else, that employer will also have to submit a new petition for you through USCIS. That may well be difficult as I imagine you know. Still, it is at least possible. If you leave the US, you’ll need a new H1b visa to return, showing your current employer. Check out uscis.gov for more information.