Signed into my account on my dad's laptop. Data was lost and merged with mine?
All his bookmarks and saved passwords were merged into my Firefox account and deleted from his laptop when I signed in from my PC. Is there any way to undo this and return/restore the settings on his laptop?
Opaite Mbohovái (2)
If you used his profle or account with FireFox you wiped clean anything he had for bookmarks and logins. There is no fix for this unless you can restore his old profile back and if there nothing to restore then your going to have to figure how your going to fix this.
If these don't work getting his profile back then your going to have to explain to him.
This is why you should Never sync anyone computer if he didn't create another profile for you to use FireFox to Sync to.
You should lose data if you sign in to Sync, but you should never sign in to your account on a device that isn't your own as that will cause data from both devices to get merged like you noticed. You can try to disconnect from Sync on your dad's device and remove synced data. Best would be to backup your profile data on both devices to be sure.
For the bookmarks there are automatically created backup in the bookmarkbackups folder.