Recently, Firefox WebExtension stubs (November 2017) have been added firefox-webext-browser. The steps in the answer that you linked are now valid (the corresponding changes are shown in bold):
First time setting
Settings (File > Settings)Languages & Frameworks > Javascript > LibrariesDownload- ,
TypeScript community stubs firefox-webext-browser ( , firefox-w)Download and InstallOK, .
Settings (File > Settings)Languages & Frameworks > Javascript > Libraries@types/firefox-webext-browserOK, .