Lewis Rosenthal has been been working with Philip Chee of xSidebar fame, adding SeaMonkey support to some Firefox and Thunderbird extensions. They should show up on xSidebar in time, but Lewis has some available (1/20/2013) on his FTP server, including Abduction! version 3.0.17, BetterPrivacy version 1.68, Evernote Web Clipper version 5.5, and FireFTP version 2.0.7.
Source: OS/2 Warp news and rumors