Fix Firefox 91 tabs

userChrome.css

/*
On Mac, paste this into ~/Library/Application Support/Firefox/Profiles/5a6b7cx1.default-release/chrome/userChrome.css
The random ID before default-release will be different for you.
You will have to create the chrome directory and the userChrome.css file.
To tell Firefox to use userChrome.css, follow the instructions here:
https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/
You must restart Firefox for changes to this file to take effect.
*/

/* Remove rounded corners from everything */
* {
  border-radius: 0 !important;
}

/* Adjust menu padding; 4px makes the menus a bit denser */
menupopup > menuitem, menupopup > menu {
  padding-block: 4px !important;
  padding: 4px !important;
}
:root {
  --arrowpanel-menuitem-padding: 4px 8px !important;
}

/* Make the title bar (containing the tabs and the close/minimize/maximize buttons) dark */
#titlebar {
  background-color: #444;
}

#urlbar-input-container {
  border: 1px solid #ccc;
  background: #fff;
}

@media (-moz-proton) {
    /* Format the tabs as tabs instead of floating buttons */
    .tab-background {
        margin-block: 0 0 !important;
    }

    /* Separate tabs with a thin white line */
    .tabbrowser-tab:not([selected=true]):not([multiselected=true]):not([beforeselected-visible="true"]) .tab-background {
        border-right: 1px solid rgba(255, 255, 255, .20) !important;
    }

    /* Give the active tab a blue top border */
    .tabbrowser-tab[selected=true] {
        border-top: 2px solid #08f !important;
    }

    .tabbrowser-tab[selected=true] .close-icon,
    .tabbrowser-tab[selected=true] .tab-secondary-label,
    .tabbrowser-tab[selected=true] .tab-icon-overlay {
      color: #000 !important;
    }

    /* Remove borders from inactive tabs */
    .tabbrowser-tab:not([selected=true]):not([multiselected=true]) {
        border-top: none !important;
    }

    /* Inactive tabs have a dark background */
    .tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-background {
        border-radius: 0 !important; /* FIXME remove */
        background-color: #444;
    }

    .tabbrowser-tab:not([selected=true]):not([multiselected=true]) .close-icon,
    .tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-secondary-label,
    .tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-icon-overlay {
        color: #fff !important;
    }

    /* Inactive tabs have light text */
    .tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-text {
        color: #fff;
    }

    /* Hovering over an inactive tab brightens the background slightly and highlights the close button */
    .tabbrowser-tab:not([selected=true]):not([multiselected=true]):hover .tab-background {
        background-color: #555 !important;
    }

    /* Remove padding between tabs */
    .tabbrowser-tab {
        padding-left: 0 !important;
        padding-right: 0 !important;
        padding-top: 0 !important;
        margin: 0 !important;
        border-bottom: none !important;
        border-left: none !important;
        border-right: none !important;
        border-top: 2px solid #08f;
    }
}