If you installed from a package manager, the package manager typically handles updates. Can you check on the Ubuntu forums on how to control when the package manager updates Firefox.

some tabs (youtube) stop working without any indication why

The AdBlocker Ultimate extension is causing issues for many users recently. If you have it installed you can:

• Disable it.
• Uninstall it from about:addons, refresh the page to clear settings then re-install.
• Use an alternative like uBlock Origin.

There is no "Check for Updates in Settings > Updates, nor is there "Allow Firefox to" So I cannot change any Update settings. ATM Firefox (now version 116) updates automatically and some tabs (youtube) stop working without any indication why. I would like to disable automatic updates but I can't. (AFAIK) Why?

If you installed from a package manager, the package manager typically handles updates. Can you check on the Ubuntu forums on how to control when the package manager updates Firefox.

As jscher2000 said, you installed a third-party version from Ubuntu so the updates are provided by them.

The internal Firefox updates only work with the official builds of Firefox from mozilla.org like say https://www.mozilla.org/firefox/all/#product-desktop-release as Mozilla does not provided updates to Firefox builds they did not well build.