Using the new WebExtensions API's, it is not possible to display multiple tabs simultaneously within the same browser window. This is why Tile Tabs WE has to move each tiled tab into a separate window.

Reasons why you might want to use Tile Tabs WE:

• To compare multiple web pages side-by-side.
• To synchronize scroll multiple web pages side-by-side.
• To open links from one tab in adjacent existing tabs.
• To save tiled layouts which can be re-opened for future use.

UPDATE:

Tile Pages WE has just been released and displays multiple web pages in a single browser window!