Or accidentally dragged it into another folder, as a friend of mine did with the complete Applications folder, as a result of which he came to me in a state of hair-tearing.

Against that possibility as an explanation is that the alias system should be capable of finding a target app. (or a file) anywhere on the same drive, or the same volume if there is more than one volume on a drive—even if the target's name has been changed—because an alias points to a unique number assigned to the target, not a name.

Another, and perhaps more likely possibility is that an auxiliary file of FireFox is damaged or missing.

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