minMaxCloseButton.uc.js

// ==UserScript==
// @name            MinMaxClose Button
// @author          xiaoxiaoflood
// @include         main
// @shutdown        UC.MinMaxCloseButton.destroy();
// @onlyonce
// ==/UserScript==

// inspired by: https://j.mozest.com/ucscript/script/83.meta.js

UC.MinMaxCloseButton = {
  init: function () {
    _uc.sss.loadAndRegisterSheet(this.STYLE, _uc.sss.USER_SHEET);
    const { CustomizableUI } = window;
    CustomizableUI.createWidget({
      id: 'minMaxClose-button',
      type: 'custom',
      defaultArea: CustomizableUI.AREA_NAVBAR,
      onBuild: function (doc) {
        let btn = _uc.createElement(doc, 'toolbarbutton', {
          id: 'minMaxClose-button',
          class: 'toolbarbutton-1 chromeclass-toolbar-additional',
          label: 'Window Button',
          tooltiptext: 'Left-Click: Minimize\nMiddle-Click: Maximize/Restore to previous position\nRight-Click: Exit',
          oncontextmenu: 'return false',
          onclick: 'UC.MinMaxCloseButton.BrowserManipulateCombine(event)'
        });

        return btn;
      }
    });
  },

  BrowserManipulateCombine: function (e) {
    let win = e.view;
    switch (e.button) {
      case 0:
        win.minimize();
        break;
      case 1:
        let max = win.document.getElementById('main-window').getAttribute('sizemode') == 'maximized' ? true : false;
        if (max) {
          win.restore();
        } else {
          win.maximize();
        }
        break;
      case 2:
        win.BrowserTryToCloseWindow();
    }
  },

  STYLE: Services.io.newURI('data:text/css;charset=UTF-8,' + encodeURIComponent(`
    @-moz-document url('${_uc.BROWSERCHROME}') {
      @media (prefers-color-scheme: dark) {
        #minMaxClose-button {
          list-style-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAAa0lEQVQ4T72SQQ4AIQgD5f+PVjnUQ7fdZUMiNw2ZjkLMXYMqdvGdO0cb4MgKnL1sZ1XbAGWW0E+Dt+R7ANZEctnAAdS/PKagUtQdYBIgk8x2llcW08Hzzrm689nHI05Y2QBBbPIL0DJwf7AAeTpgCcCBOFsAAAAASUVORK5CYII=');
        }
      }
      @media (prefers-color-scheme: light) {
        #minMaxClose-button {
          list-style-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAAO0lEQVQ4jWNgYGD4jwWTBCg2gBSDSTKcYgNwGUqRzfQzAJcYxQYQBahiAE0SF7pBVEn2VHEJ/VyANQwACylDvQ9eqkEAAAAASUVORK5CYII=');
        }
      }
    }
  `)),

  destroy: function () {
    Services.wm.getMostRecentBrowserWindow().CustomizableUI.destroyWidget('minMaxClose-button');
    _uc.sss.unregisterSheet(this.STYLE, _uc.sss.USER_SHEET);
    delete UC.MinMaxCloseButton;
  }
}

UC.MinMaxCloseButton.init();

Simplified version of MinMaxClose Button from https://github.com/xiaoxiaoflood/firefox-scripts

Middle-Click will now just restore/maximize window, instead of having extended feature of restoring to certain hard-coded values.