Fantastic plugin. I eventually used my profile's userChrome.css to hide Firefox's horizontal tab row because I just don't use it anymore, and I much prefer Sidebery's use of tab panels over Floorp's use of workspaces (not to mention Sidebery + Firefox don't crash on me constantly, or ever).

Of note: if/when customizing the size of tabs height in Sidebery's style settings, stick to px and don't use relative units like ems, as this will mess up your drag & drop interactions.