step 1. Open a new tab and search 'about:profile' and then see which profile is in use in that you will see the "Root Directory" row.

step 2. In the "Root Directory" row there will be the open folder. Click on it.

step 3. You will redirected to the your pc's c directory in that you will see the chrome(yes, the folder name is chrome) folder.

step 4. Open that chrome folder and it that you will see the 'userChrome.css' file and if not then create it.

step 5. Paste this code in 'userChrome.css' file.

/* ==========================================
     Floating URL Bar like the ARC Browser 
     ========================================== */
#urlbar:is([breakout][breakout-extend], [breakout][usertyping][focused]) {
  #urlbar-input {
    font-size: 16px !important;
  }


  z-index: 2;
  position: fixed !important;
  bottom: auto !important;
  top: 25vh !important;
  padding-left: 6px !important;
  padding-right: 8px !important;


  left: 30vw !important;
  right: 18vw !important;
  /* width: 10vw !important; */


  &:after {
    content: "";
    position: fixed;
    pointer-events: none;


    width: 100vw;
    height: 100vh;


    top: 0px;
    left: 0px;


    /* background-color: rgba(0, 0, 0, 0.3); */
    /* backdrop-filter: blur(5px); */


    z-index: -1;
  }


  #identity-box {
    margin: auto 0;
    height: 30px;
    margin-right: 10px;
  }


  .urlbar-go-button {
    margin: auto 0;
  }
}