Tile Tabs

Tile Tabs allows tabs to be displayed in tiled layouts within the Firefox tabbed browser content area. In each tiled layout, tiles can be arranged horizontally, vertically or in a grid, and tiles can be re-sized by dragging splitter bars. Tabs and links can be dragged to open in new or existing tiles. The scrolling of tiles can be synchronized. A tiled layout can display tabs from more than one Firefox tab group.

- from the Mozilla add-on page.