Using with Firefox 7 Rated 4 out of 5 stars
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-220.127.116.11-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.
Updating version compatibility Rated 4 out of 5 stars
OpenBook 18.104.22.168 fixes one of the most glaring errors in the chrome of Firefox, a problem not existing in SeaMonkey. It works fine with Firefox 3.6.3 if you do a little 'update engineering' before installing.
Download the openbook-22.214.171.124-fx.xpi file, open it and extract the install.rdf file, edit it to change em:maxVersion="3.1a2pre" to em:maxVersion="3.6+", reinsert install.rdf into the .xpi file, and then install it in Firefox (drag the .xpi into a Firefox window).
Since OpenBook 126.96.36.199 works fine in Firefox 3.6.3 and is so helpful, I don't understand why the author hasn't made this maxVersion update in the posted .xpi file so users don't have to get so creative. I would have given OpenBook five stars except for this, maybe six.