You can't stop the updates. There are steps one can do but it's not discussed here. If Firefox needs to restart because of update you don't have a choice it will have to restart to insure the updates works. And doing so will sign you out of any sites your logged in for security reasons. What you want to do is close once it updates and start the browser but do a cntl-shft-t to restore the session tabs but if you close before doing that all the sessions will be gone. And if you have "remember me" setup then your login are set for those sites to confirm the login to access them again.