Rated 3 out of 5 stars

Tested in Windows7. The plugin is unstable. The browser starts to slow down.
Why you can not do Split tab within the same browser window?

This review is for a previous version of the add-on (2.0.2011062701.1-signed).  This user has a previous review of this add-on.

Why this is redesigned based on multiple windows instead of subframes in a window?

1) I think that each "pane" should have navigation toolbar.
2) Old Fox Splitter actuary did it, however, very long codes were required and it had too less maintainability.
3) So I couldn't update the old Fox Splitter for 1.5 years.
4) During that time, similar new addons "Tile Tabs" and "Tile View" debuted. I hope that they can replace my old Fox Splitter.
5) However, after I tried them, I couldn't become familiar with their concept. I thought that their "tiled view in a tabbrowser" is hard-to-understand.
6) And, both "Tile Tabs" and "Tile View" didn't provide navigation toolbar for each pane. I realized that it was a unique feature of my old Fox Splitter and no other addon inherited the concept. I thought that I have to implement such an extension by myself if I want to use it -- no one does it for me except myself.
7) I decided to update Fox Splitter.
8) I don't want to abandon the new Fox Splitter for a long time. High maintainability (and compatibility) code is strongly required for me. The old Fox Splitter didn't have them and I cannot cover them by my crazy hacking time - now I have less time to do it.

They are the reason why I designed Fox Splitter based on multiple windows.
I recommend you to try Tile Tabs ( https://addons.mozilla.org/firefox/addon/tile-tabs/ ) or Tile View ( https://addons.mozilla.org/firefox/addon/tile-view/ ).