Hi I am guessing that you are referring to your Firefox Account? If you see devices there that you go not recognise, you can use the controls on that page to remove them. You may also want to add an additional layer of security to your account. This support article should help: https://support.mozilla.org/en-US/kb/secure-firefox-account-two-step-authentication#:~:text=Click%20Firefox%20accounts%20from%20the,next%20to%20Two%2Dstep%20authentication.