I have been working on a Firefox extension which will provide multiple workspaces within a single window.
You can find the source repository here.
To download version 0.2 Right click > Save link as here.
Note that version 0.2 supports only 2 workspaces. If you're unhappy with that try Version 0.3rc1 is available here.
I have tried 0.3rc1 on Firefox version 3.0.6. Version 0.2 is known to work in 3.0.6 and 3.5.6 (I hope it implies that it will work in all intermediate releases).
If you are interested you are welcome to test/enhance the extension. For any bug reports/queries/suggestions contact me.
A TODO list should be up shortly.