Isolate Access to History navigation to Tor state preference does not work

Reading the call to test the new Torbutton version (and Mike's thinking about a kind of a bug bounty ;) ) I looked a bit around and found that 5.12 of the Torbutton design documentation is violated.

To test this

1. Create a new profile with the latest alpha version of Torbutton.
2. Start Firefox (the problem occurs on FF 3.6.16 and 4.0) and activate Torbutton (if it is not already active).
3. Load e.g. http://ip-check.info
4. Load afterwards e.g. http://www.heise.de
5. Toggle the Tor button.
6. Press the back button and you'll see probably your real IP.

Reproducible: Probably always (on my machine an Ubuntu 10.04 it worked pretty well)

Expected results: Get the fancy dialog shown that moving back to the previous domain in a different Tor state is disallowed due to some Firefox bugs.

I tested it shortly once with Torbutton 1.2.5 and the expected dialog popped up.