Mac OS X 10.5 Leopard , Mac OS X 10.6 Snow Leopard , Mac OS X 10.7 Lion , Mac OS X 10.8 Mountain Lion , Mac OS X 10.9 Mavericks , Mac OS X 10.10 YosemiteWindows XP , Windows Vista , Windows 7 (Seven) , Windows 8 , Windows 8.1 , Windows 10
This plug-in is called "Tile Tabs". IT is a handy Firefox add-on which displays some or all of your tabs in a tiled layout, allowing you to view them all at once. To try this out, open three or four tabs and click Tile > New Layout (or use the button on your address bar).
The menu has a lengthy list of layout options - 2, 3, 4 or all tabs, tiled vertically, horizontally, or in various grids - but if you just click All Tabs - Horizontal you'll see all your opened tabs tiled one above the other.