very good Rated 5 out of 5 stars

First of all, congratulations to developer for his efforts!

Now, i think that there is a small distance to be perfect..
In my opinion, the option "Open in next" should be redefined to be more clear. In what "Next"??
About its actual functionality, i don't like it in current version (6.1).
Upon its appearance and until the previous version (6.0) i was very happy
because i could have a layout of two vertical tiles side by side to open links from
one tile to other. Now i can't any more! :(
For example, think about a tile with youtube or google search results to be
open in the other tile.
It was very convenient to me!
So i suggest to developer that, in combination with this feature, should be also another one
to set the behavior (e.g. open in next tile or in new tab).
This way would be happy all users!

Now, about the snap feature of splitters, i don't understand what's the usefulness
the splitters to be able to snap in edges.
In my opinion, it would be far more useful if was possible, dragging a splitter to reach
the splitter of another tile, to automatically be aligned with it reaching the specified
distance. I know that this can be done by double clicking on a splitter but, the
desirable position of the splitters is not necessarily always the center of tiles.

A restored layout from a previous session when firefox starts, breaks the
functionality of firefox "Don't load tabs until selected". Also, in such a case, some
of the inactive tiles show a weird string instead of their content until be selected.
This also applies when one of the inactive tiles contains the extension manager.
As i realize, according to the expected behavior in a restored layout, only the
active tile should be loaded. Otherwise, in a restored layout of several tiles, we
can't utilize the benefit of "Don't load tabs until selected".

I encourage the developer to find good solutions for the above for this invaluable
extension . . ! ! :)

This review is for a previous version of the add-on (6.1). 

Thanks for these suggestions

Open in Next Tile - There is an option to configure this in 6.2.

Splitter Snap - The requirement for the splitter snap operation is not very clear. Please e-mail more details (and snapshots) to dw-dev@gmx.com.

Restore Layout - When restoring a layout, does it really make sense to delay loading of tabs until they are selected? With normal Firefox, you can only see one tab's contents at any one time. Tile Tabs fundamentally changes the paradigm, so you can see the contents of multiple tabs. The whole point is that you can see multiple tabs simultaneously (not empty tiles). Any tabs that are not currently displayed in the tile layout, will not be loaded until selected. Similar arguments apply if loading a previously saved tile layout (that was saved with its tabs).