Why can't I find Firefox in the AppCenter?
Not only Firefox, but also other non-elementary Apps.
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Sign up to join this communityAFAIK the AppCenter only shows apps whitin the Ubuntu Store and sources. If those apps you have aren't on the official repos they won't show up there.
That said, Firefox appears in my AppCentre.
Try
sudo apt update
This may be related to an unfixed bug:
AppCenter is empty if apt update has never been run [$30]
Expectation: AppCenter fires up packagekitd and updates the software catalog transparently in the background when the session starts up
Reality: AppCenter only shows data for the pre-installed packages. packagekitd never fixes it. AppCenter is useless until you open a Terminal and manually run apt update