Their download page doesn't make this clear: Molly is not on F-Droid.

Instead, the Molly project hosts an F-Droid-compatible repository, which you can configure your F-Droid client to use in addition to / instead of the F-Droid repository. If you do this, the downloaded software will come directly from the Molly developers, not from F-Droid.

Some people avoid this because it loses a layer of oversight. Others prefer it because it avoids a potential attack vector. You'll have to decide for yourself whether it's something you want to do.

Add their repository: https://molly.im/fdroid/foss/fdroid/repo

Or you could get it directly from their github releases using obtainium if you don't like to mix repos in to fdroid.

It's listed on Accrescent

Add this repository to the F-Droid app.