Pop!OS is a pretty stable distro, so there shouldn’t be any dependencies problem: you usually have that on arch or arch based distros, even if it’s quite rare. Have you ever uninstalled Firefox and reinstalled it but without some of its dependencies? Or ignored a package it asked you to install with it afterwards? If not, maybe it’s just an update that went wrong, like the PC shut down during an update or something. So update your system, uninstall firefox and reinstall. Your data stored in your home directory (in .mozilla, a hidden folder) like your history, extensions, opened tabs, should be conserved, even if you use apt purge.