I would be an amateur, at writing a Firefox bug-report -- I hope that that is O.K., with the developers .

I cannot seem to nail-down the circumstances, upon which format, the dialog-box appears, on the Mac .

Everyone's an amateur the first few times. :-)

Do you use Firefox Sync? Normally you're prompted when Sync wants to run the first time and if you cancel, then at some later time you may be prompted again (probably after many hours). Otherwise, the dialog should only be triggered by Firefox recognizing a website login form it wants to fill.

Yes -- I think that I set-up, a Firefox Sync account, although I'm darned, if I remember the password, for it .

rsblanchard said

Yes -- I think that I set-up, a Firefox Sync account, although I'm darned, if I remember the password, for it .

It would commonly be saved in the Password Manager, so you have that at least.

But -- I tried to look-it-up, in the "Saved Logins", and, apparently, it was NOT in clear-text .

rsblanchard said

But -- I tried to look-it-up, in the "Saved Logins", and, apparently, it was NOT in clear-text .

Hmm, that makes sense, I guess. If you have a complete set of data -- local backups are a good idea -- then you can reset the Sync password to flush the account and start again if there's no other alternative. Probably better to do that now than wait until it's too late and you really need it.

See: I've lost my Firefox Sync account information - What to do