Since you are using Linux I am guessing you are using a package build of Firefox provided by Mint? The Firefox updates are in your package manager. https://blog.linuxmint.com/?p=4244 Though if you are not able to get anymore package updates including Firefox, then your version of Mint may be EOL in packages support. Mozilla only provides updates to their own builds of Firefox for Windows, macOS, and Linux https://www.mozilla.org/firefox/all/desktop-release/ and not to third-party builds.