Hi folks, thanks for reporting and testing, but what I suspected is what is surely the case. My bad, I had forgot to put the updating of the DEBINX data on our server for dCore-focal64 in the script that syncs our DEBINX to Debian/Ubuntu Packages files automatically. The script that I run manually had it in here, but not the automatic one. Please try "sce-import firefox" again, and if you have an SCE directory, import and run sce-update to update the entire directory, that is important too.
sce-import sce-update
sce-update -an
I tested the importing and running of firefox on a fresh dCorePlus-focal64 usb install and all is good now. All those other package choices related to firefox that you see when running "sce-import firefox" are different locales and all, just plain firefox is the main one. Also, there is the package dCore-firefox-installer that can be ran and be used to download and create a firefox-latest package of the latest firefox from the Mozilla site.
Also, if a menu pops up during the downloading of the .deb packages in an sce-import session asking you to choose which .deb version, then something is wrong. It may be an update Debian/Ubuntu made since our syncing of DEBINX data, so try again in a couple hours. If the menu still pops up asking you to choose which .deb for a package, then please report it here. Thanks.
Sce-remove - it is now an extension. Anything that can be an extension is one, to save having to upload a lot of data and create another release candidate in the event of a change in that utility.