If you are using Tor or Librewolf, it is probably “letterboxing”, a part of ResistFingerprinting, which you should find in about:config or in the settings.
… but like the other person implied, its not a great idea to disable this. I would like to recommend checking out the EFF’s page on the topic – https://coveryourtracks.eff.org/
But you are an adult, presumably, so I will leave you the information and let you decide. Be safe out there
Depressing :(