According to https://endoflife.date/linuxmint Linux Mint 21 is still being supported so if you are using a deb package build of Firefox then there is likely update in the package manager. With Mint there are five ways to have Firefox installed. As a deb, from Linux Mint repository, as a flatpak, snap and as a deb from Mozilla repository and of course the Linux tarball from Mozilla site https://www.mozilla.org/firefox/all which I prefer. The tarball version gets internal Firefox updates like macOS and Windows versions does even if the Linux distro is eol in repository updates. https://forums.linuxmint.com/viewtopic.php?p=2483742&sid=3051368ac25f2ce7d7d2347b12bcb87b#p2483742