Always on Top

Simple add the <songbird:onTop> element to your songbird:compatibleLayout section in your Feathers' install.rdf. For example:

<songbird:feathers>
    <Description>
        <songbird:skin>
            <Description>
                <songbird:name>Sidecar</songbird:name>
                <songbird:internalName>sidecar</songbird:internalName>
                <songbird:compatibleLayout>
                <Description>
                    <songbird:layoutURL>chrome://sidecar/content/xul/mainplayer.
xul</songbird:layoutURL>
                    <songbird:showChrome>true</songbird:showChrome>
                    <songbird:onTop>false</songbird:onTop>
                </Description>
                </songbird:compatibleLayout>
            </Description>
        </songbird:skin>

Declaring <songbird:onTop>false</songbird:onTop> will set the default behaviour of not being on top, while declaring <songbird:onTop>true</songbird:onTop> will set your Feather to be always on top.