Hi,
To pin Site you can click on the pin icon on the thumbnail - it should become red once pinned. That's one way, but that means the site needs to be already in the list.
The other is to what you say with Tack box. I just tried it with:
http://www.google.com and it worked. You have to put http:// in front though. Also, you need to open new tab to see the change.
Sites are not randomly populated, the list is based on most recently and most frequently accessed sites (from browser history). The list is updated on Firefox re-start so if you're like me and keep your browser opened for days it'll keep the list it started with. If you clear your history it won't know which of your sites you regularly visit so it will appear random for a while until the history is built up.
Unfortunately I'm not actively developing the extension any more.
Cheers,
Mihailo