Firefox can use Emacs key bindings in editable text fields, but the method for enabling this feature has changed in Mint over the years. In the Mate edition of Linux Mint, this command will do the trick:
gsettings set org.mate.interface gtk-key-theme 'Emacs'
In the Cinnamon edition of Linux Mint, use this:
gsettings set org.cinnamon.desktop.interface gtk-key-theme 'Emacs'