This is the same issue and the same answer as in /questions/1538103.

While the other problem was fixed with the given solution, it did not fix the problem with the collections. I can still not see the collections on the homepage and have no way to access them, even tough I know they are there (I see them when I add new pages to a collection).

This also happened to me. Heaven only knows what caused it but my extensive list of collections went awol, never to return. They were visible when I went to save a new item but always vanished immediately after.

After trying every combination of settings and security I reluctantly (knowing full well I'd loose everything) decided to clear the Firefox cache and program memory but even that didn't reinstate the ability to see newly created collections. So an uninstall - restart - install was needed to get Firefox working properly... until the next time! I'll not be using Collections again until this serious bug is acknowledged and fixed... in fact I might stop using Firefox (Android) altogether.

I managed to find a cause and a solution. I found that there is Debug mode (go to About Firefox menu and press on logo several times) that opens a new section called Nimbus experiments. Apparently there is a switch there called "Simplify Homescreen New-users 145.0.1": if it's set to "Control", collections are back there. If it's on "treatment-a", they're gone. (You need to reload the browser for changes to take effect).

So it's some kind of experiment to change the interface, but I don't understand why there's no option to enable/disable Collections manually. I used this feature for bookmarks, it is so much more useful than Shortcuts because you can have more websites pinned. But without it, opening websites takes more time.