how to do the pair a device on firefox?, what's that for? and i dont know how to do it?

[DunbarPappy®ϟϟ]

You've got 2 distinct concepts to deal with:
1- passwords;
2- sync

1- Firefox does NOT have a "Master password" by default.
If you utilize this password saving feature, you must create a bullet proof "Master" passphrase. Keep that secret.
If you don't create one, then anyone can go to the 'Saved passwords' button and see what those are, in plain text. (Chrome by the way has an even worse arrangement).

2-Sync I have not used, but I believe it's where you are migrating one devices Firefox settings, bookmarks, etc., to another different device, also having Firefox.
It uploads your Ff 'stuff' to Mozilla servers; then you download them to that 2nd device.
Read the Mozilla site on that to make sure on the steps.
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A better approach to this entire problem would be to have separate computer 'User Accounts'. This would resolve many issues that otherwise will plague you & your brother in the future.
Look in Control Panel> User accounts, and 'create a new account'. (Requires Admin. privileges.)
Both computer accounts would likewise need bullet proof passwords.
You should still use the concept outlined in #1 though.