u/SuffocatingDensity

The great thing about Firefox is that if there's something you don't like, you can change.

I personally like the new Nova UI, but I'm not a fan of some of the colors, so I added some code to my userChrome to fix it.

If the rounded corners are bothering you, use the Browser Toolbox to identify the element you want to change, and change! You can even have Claude Code or ChatGPT do it, it's that easy!

Here's a snippet from my userChrome that removes the tab borders, changes their color to match the system colors, and removes the theme gradient.

@media(prefers-color-scheme: light) {
  /* Selected tab appearance */
  .tabbrowser-tab[selected] .tab-background {
    --tab-background-color-selected: #f8f8f8 !important;
    --tab-border-color-selected: #dcdcdc !important;
  }

  /* Light theme surfaces */
  #sidebar-main,
  #sidebar-box,
  #sidebar-box #sidebar-header,
  #sidebar,
  #browser {
    background: #f7f7f7 !important;
  }
  
  #nav-bar,
  #PersonalToolbar,
  #TabsToolbar,
  #titlebar {
    background: transparent !important;
    background-color: #e9e9e9 !important;
  }  
}

@media (prefers-color-scheme: dark) {
  /* Selected tab appearance */
  .tabbrowser-tab[selected] .tab-background {
    --tab-background-color-selected: #515151 !important;
    --tab-border-color-selected: #5f5f5f !important;
  }
    
  #nav-bar,
  #PersonalToolbar,
  #TabsToolbar,
  #titlebar {
    background: transparent !important;
    background-color: #1e1e1e !important;
  }  
}