What you should have done was after signing out was click the three horizontal lines at the top right and choose Passwords then click the three horizontal dots at the top right of the next screen and finally 'Remove All Passwords.'

I can't find a ‘Reset Firefox’ function, only ‘Refresh Firefox,’ so can't say if it was mis-leading as to what it would do or not.

If I were selling a computer at the very least I'd do a factory re-set on it. Not only would that clear my data, though it could still be recovered by somebody with the ability, but also give the new owner a computer free of things set up how I like but they may not.

Were you using Sync and did you reconnect the new device to the same Sync account as it sounds that was what you did? Only a password reset of the Mozilla Account would wipe all data stored on the Sync server.

Note that Mac has a hand-off feature.

• https://support.apple.com/guide/mac-help/hand-off-tasks-between-devices-mchl732d3c0a/mac