The Tor Browser / file revision

 var tabstrip = gBrowser.tabContainer.mTabstrip;

 var scrollbox = tabstrip._scrollbox;

 var originalSmoothScroll = tabstrip.smoothScroll;

 var tabs = gBrowser.tabs;

 function rect(ele)           ele.getBoundingClientRect();

 function width(ele)          rect(ele).width;

 function left(ele)           rect(ele).left;

 function right(ele)          rect(ele).right;

 function isLeft(ele, msg)    is(left(ele) + tabstrip._tabMarginLeft, left(scrollbox), msg);

 function isRight(ele, msg)   is(right(ele) - tabstrip._tabMarginRight, right(scrollbox), msg);

 function elementFromPoint(x) tabstrip._elementFromPoint(x);

 function nextLeftElement()   elementFromPoint(left(scrollbox) - 1);

 function nextRightElement()  elementFromPoint(right(scrollbox) + 1);

 function firstScrollable()   tabs[gBrowser._numPinnedTabs];

 function test() {

   requestLongerTimeout(2);

   waitForExplicitFinish();

   // If the previous (or more) test finished with cleaning up the tabs,

   // there may be some pending animations. That can cause a failure of

   // this tests, so, we should test this in another stack.

   setTimeout(doTest, 0);

 }

 function doTest() {

   tabstrip.smoothScroll = false;

   var tabMinWidth = parseInt(getComputedStyle(gBrowser.selectedTab, null).minWidth);

   var tabCountForOverflow = Math.ceil(width(tabstrip) / tabMinWidth * 3);

   while (tabs.length < tabCountForOverflow)

     gBrowser.addTab("about:blank", {skipAnimation: true});

   gBrowser.pinTab(tabs[0]);

   tabstrip.addEventListener("overflow", runOverflowTests, false);

 }

 function runOverflowTests(aEvent) {

   if (aEvent.detail != 1)

     return;

   tabstrip.removeEventListener("overflow", runOverflowTests, false);

   var upButton = tabstrip._scrollButtonUp;

   var downButton = tabstrip._scrollButtonDown;

   var element;

   gBrowser.selectedTab = firstScrollable();

   ok(left(scrollbox) <= left(firstScrollable()), "Selecting the first tab scrolls it into view " +

      "(" + left(scrollbox) + " <= " + left(firstScrollable()) + ")");

   element = nextRightElement();

   EventUtils.synthesizeMouseAtCenter(downButton, {});

   isRight(element, "Scrolled one tab to the right with a single click");

   gBrowser.selectedTab = tabs[tabs.length - 1];

   ok(right(gBrowser.selectedTab) <= right(scrollbox), "Selecting the last tab scrolls it into view " +

      "(" + right(gBrowser.selectedTab) + " <= " + right(scrollbox) + ")");

   element = nextLeftElement();

   EventUtils.synthesizeMouse(upButton, 1, 1, {});

   isLeft(element, "Scrolled one tab to the left with a single click");

   let elementPoint = left(scrollbox) - width(scrollbox);

   element = elementFromPoint(elementPoint);

   if (elementPoint == right(element)) {

     element = element.nextSibling;

   }

   EventUtils.synthesizeMouse(upButton, 1, 1, {clickCount: 2});

   isLeft(element, "Scrolled one page of tabs with a double click");

   EventUtils.synthesizeMouse(upButton, 1, 1, {clickCount: 3});

   var firstScrollableLeft = left(firstScrollable());

   ok(left(scrollbox) <= firstScrollableLeft, "Scrolled to the start with a triple click " +

      "(" + left(scrollbox) + " <= " + firstScrollableLeft + ")");

   for (var i = 2; i; i--)

     EventUtils.synthesizeWheel(scrollbox, 1, 1, { deltaX: -1.0, deltaMode: WheelEvent.DOM_DELTA_LINE });

   is(left(firstScrollable()), firstScrollableLeft, "Remained at the start with the mouse wheel");

   element = nextRightElement();

   EventUtils.synthesizeWheel(scrollbox, 1, 1, { deltaX: 1.0, deltaMode: WheelEvent.DOM_DELTA_LINE});

   isRight(element, "Scrolled one tab to the right with the mouse wheel");

   while (tabs.length > 1)

     gBrowser.removeTab(tabs[0]);

   tabstrip.smoothScroll = originalSmoothScroll;

   finish();

 }