I tried to use this with (and without TreeStyleTabs) so I can get the bookmarks alongside with my left sidebar where my tabs are located and could somehow manage to make it work for a few hours before it glitched out and disappeared completely as an IFRAME. (only the window option worked but since it's ugly and impractical, I didn't use it)
When it "worked" it would usually be very glitchy on youtube and cover the left part of the video (unless in fullscreen. )
Some sites wouldn't show the IFRAME at all for some reason.
It would also hog resources and eventually freeze FF to a point I had to restart it.
Also, the support of drag and drop tabs to the bookmark IFRAME would be great.

EDIT 1 : Found out what it was : The "hide left bar" option is glitchy and doesn't show a way to restore it. you have to click on an extremely small and invisible part of the left of the native sidebar for it to show (I had to minimize it so youtube showed properly)
EDIT 2 : I also found out that many other websites are getting messed up by this add-on, wether or not you use an I-frame it seems those sites (battle.net, for instance) shrinks the page so it can put an I-frame even if it's set to disabled, wasting a lot of useful screen space in the process.

Overall, it's a step in the right direction, but the many bugs make it hardly usable for me.
I just wish FF had a way to display 2 sidebars at the same time natively.