Tile Tabs 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.
This isn't just some static view of your tabs. Each tile is effectively a separate browser, just as before, so you can click links and browse as normal in one tile, while the others remain visible.
Alternatively, if these tiles are displaying related content, you might press F8 (or click Tile > Sync Scroll) to link them all together. Now scroll one tab and the others scroll, too, great when you're comparing resources.
In this example we chose to tile all tabs, which is quite drastic (every tab then displays the tiled view). A much simpler alternative would be to click Tile > Tile New Tab > Below. This splits your current tab into two tiles, with the current page at the top, your "new tab" page below, while all other tabs remain untouched.
Fortunately, whatever you're doing, you can close tile layouts at any time to restore your original tabs.