Incidentally, aside from the fancy extension installer method, there are two other bookmark methods to make pages load in the sidebar.
You can go into Bookmarks->Manage Bookmarks... (or just pull down the bookmark menu) then pick the one you want and right-click, select Properties, then check the box that says Load this bookmark in the sidebar, click OK.
If you want to programmatically add a bookmark from a page that will open in the sidebar, use e.g.
The user will get an "Add Bookmark" popup window, and whenever they click on that bookmark, it will open in the sidebar.
Info based on Mozilla Sidebar.