I use Tree Style Tab -- it's actually one of the main reasons I use Firefox -- and this does make me a bit nervous. TST and its competitor Sidebery are both Firefox "Recommended Extensions"[1], which receive extra scrutiny from Firefox staff. I'd like to think that these extensions are as trustworthy as the browser itself, but the Firefox article doesn't go into great detail about how (and how frequently) they review these extensions.

[1] https://support.mozilla.org/en-US/kb/recommended-extensions-...