I found the solution! It’s not what I expected at all! I was thinking about how-and-where Firefox stores credit cards in Manjaro KDE Plasma Linux. On looking at the “modified” times of files in “~/.mozilla/firefox” in another computer, I could not find a modification time corresponding to the moment of adding a credit card. So, it appears that credit cards were being stored elsewhere, encrypted. What kind of encryption was on my computers? Well, the computers where credit card storage was working correctly had “KDE Wallet subsystem” enabled. The one computer where credit cards could not be stored had “KDE Wallet subsystem” disabled. So I enabled “KDE Wallet subsystem” and checked the “Use KWallet for the Secret Service interface” checkbox, and voila, I can now add and edit credit cards.
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