With either of your your FF's as you mouse over the mailbox folder names on the left, by any chance do you see the letters "Ed" just under and to the right the three vertical dots? I use Mac OS X FF 48.0.2 and I see that. Already submitted a ticket on it.
I think the "Ed" is the first two letters of the word "Edit" cut off by the divider.
Note I don't see it in Chrome (also equally old).
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