OK, I've got it working. I installed it on a fresh clean profile with no other add-on's initially. I had to restart the browser once after adding Straitjacket and then restart once again after I started trying to add new sandboxes, or the context menu options wouldn't show up or work to allow sandbox switching.Thanks, I don't even have published binaries but the source is here. If you check it out using Git, just zip up the contents of the src folder, rename the file with a .xpi extension and it should install.
Unlike Priv8, with this version of Straitjacket it appears I have to have some content in the window in order to switch to a new sandbox from the tab context menu - it won't just assign a sandbox to an existing blank tab. Or maybe it tries and fails to assign a new sandbox to a new blank tab. However, if I go to the Tools menu and tell Straitjacket to start a new sandbox tab, that works.
Seems to be working quite well - I really like the settings dialog, that's so much better.