I can't update this

You appear to have posted this with a old Firefox 81.0 version as your UserAgent shows Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:81.0) Gecko/20100101 Firefox/81.0 from Sept 2020. If you are not able to get new Firefox package updates in your package manager then your version of Linux distro (say Ubuntu or Mint or such) is likely End of Life or EOL in software package updates. To use the current versions of Firefox you will either have to update to a current supported version of Linux distro or you could use the Firefox Linux tarball from Mozilla. https://www.firefox.com/download/all/ However if your PC is rather old with a 32-bit CPU then 32-bit Linux support was dropped as of Firefox 145.0 and later so it would be best to use Firefox 140 ESR. I mention this because the UserAgent or UA no longer shows if you are using 32-bit and shows you have 64-bit Firefox even on a 32-bit system. https://support.mozilla.org/en-US/kb/firefox-has-ended-support-32-bit-linux https://support.mozilla.org/en-US/kb/install-firefox-linux