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.

With the regular Firefox release, there is a major update every 4 weeks. Then in between there are one, two, or three other updates to fix bugs in the major update. It can be a lot, even if you set Firefox to tell you when updates are available so you can choose when to install them.

If you prefer to use a more stable version, you could look at the Extended Support Release of Firefox 128. There are only security updates and the occasional minor bug fix. More info: Choose a Firefox update channel.

If you are using Firefox Sync, you can install the ESR alongside the regular release to transfer data before making Firefox ESR your default browser.

Why are there so many updates for Firefox?

Best case scenario there are only the major Releases. However there can sometimes be minor updates warranted for stability and or security fixes that cannot wait. Most times there has only been one minor update, occasionally two or three.