How to update my mozilla firefox?

It appears you are using Ubuntu package build of Firefox 112.0 Unless it changed since November, the web whatsapp site is looking for Firefox 115.0 and later to use it as they used to allow people with rather old versions before. Being a Ubuntu package build and 112.0 I would say your Linux distro version is likely EOL is software package updates. You will have to update your Linux distro to a current supported version or perhaps use the Linux tarball version of Firefox from Mozilla. The tarball version does get internal updates from Mozilla just like the macOS and Windows versions even if your Linux distro no longer get software package updates. Though you need to make sure wherever you put the extracted Firefox folder that it has read/write permissions for the user to get internal Firefox updates. An easy way is to have Firefox folder in /Home .. https://www.firefox.com/download/all/