The Firebird "Web Panels" feature is getting really nice, really fast. Take a look.
Web Panels is much more useful/versitile/practical than the old SeaMonkey sidebar. If you're visiting a page that you want to hold onto for a few minutes you can just pres the "Grab Page" button and it pulls that page from the content area into the Web Panels sidebar. If it's something really useful, like Gemal's BlogUpdates or the "Post to Movable Type" pop-up, then you just "Grab" it and click "Add Web Panel". That creates a bookmark for the page with a special flag that causes it to always open in the Web Panels sidebar. (You can set this flag on any bookmark by opening the bookmark's properties and checking the box labeled "Load this bookmark in the Web Panels sidebar.")
There's still more work to be done for things like context menus, supporting the SeaMonkey sidebar API, persisting content when the panel is closed or another sidebar is loaded, etc. but it's already very useful. Grab a build in the morning and take a look.
