I can't find lots of Apps in AppCenter, such as Firefox, How to install them through the AppCenter?
The app center does not handle deb packages. This is what I did and it works for me. I installed GDebi Package Installer(Cardboard box one) from the App Center. Which allowed me to install deb packages, but it removed my files manager so I went and installed another files manager called "Files" but in the description it will say its "Nautilus, also known as Files, is the default file manager of the GNOME desktop."
I got Chrome(for netflix), slack, discord, obs, Atom editor, Adobe Brackets, playsonlinux, Steam, and few more key apps installed that are not available thru the App Center. There are apps on available thru the App Center that do not mesh well with the Loki. Dolphin Files Manager, Ubuntu's Software Center, and etc. Confuses me.
Update: Found and Install Firefox thru the App Center no problem and ran it no problem too.
Well ... assuming you know how to find the download (the deb package) for what you want to install ... you download it and them open it with AppCenter ... this will either install the application or add a PPA that them let's you install the application. BTW PPA is Personal Package Archive ... basically it adds a link to where the package can be downloaded from.