While the latest version of OpenBook doesn't exactly support Firefox 7, there are a couple tricks that make it work to get around the pitiful bookmark-storing procedure of pure Firefox. As others have remarked, editing the "install.rdf" file in the "openbook-184.108.40.206-fx.xpi" install file to increase the "em:maxVersion" to something like 7.+ will permit OpenBook to install.
Then using another trick is called for. Trying to store a bookmark from the usual screen will result in the expanded OpenBook bookmark-store page showing only the top part that displays over the menu and tabs, and truncating the part that would overlay the web page display area. This makes OpenBook much less useful if where you want to store your bookmark is in the undisplayed part of the bookmark tree. To avoid this, press F11 to go full-screen, and then Ctrl+D will bring up the OpenBook display of the bookmark tree in full. It's a bit cumbersome, but surely beats the standard Firefox bookmark-store peephole.