Earlier quoted context omitted.
Tree Style Tabs was the only blocker for me, and I was amazed that it was possible to recreate under the new API. I use both Firefox and Chrome, but for very different purposes. In Chrome, I have 20-50 tabs spread across two windows. In Firefox, 400+. Tree Style Tabs is necessary for how I use Firefox. My backup plan was staying on an older version, possibly indefinitely.
400 tabs? 400 ? I'm sorry if I sound a little condescending, but have you considered using bookmarks instead? I cannot even imagine a workflow where four flippin' hundred active tabs are needed.
Bookmarks take an extra step to save to, an extra step to load from, and do not stay in sync as I browse. Synchronizing a bookmarks folder with 10-20 tab changes would take significant human overhead.
Bookmarks are also slower to review than tabs, if you need to see anything besides the name/url/icon. You would need to load the bookmark into a tab before viewing it. Tabs are already in a tab, though not necessarily loaded.