@Yann *****
Look on the VISA itself and see if it is company specific <- meaning has the company that sponsored you for the visa written on it.
IF it does have the name of the company on it you cannot use it if you find a new job and will need to go get a new visa. If it doesn't have the name of the company on it you can use it if you find a new job.
This is an example of company specific Non-B visas issued both inside the country at the immigration office and outside the country at a thai consulate