because they're inconsiderate of Tree Style Tab users. They could really well include a setting to disable the tab bar on top, but they're not doing it.
There are no security reasons for not having such a setting. None.
There's a major problem with addons doing stuff the users don't want, and the tab strip is a critical portion of the user interface so "just let them hide it" has problems with abuse. So it needs to be something that can still be autoreviewed, and it needs to be something that gives clear indications to the user as to what is going on.
They work on it. The main problem is simply that Mozilla fucked up the transition from legacy addons to the new system. There are so many APIs and functionallity still missing, that most commercial companys would have been killed over it. But with OpenSource this has become kind of normal to accept it. Just Remember KDE 3 -> 4. Python 2 -> 3. Gnome 1 -> 2 -> 3. Breaking things is just to common now.
I doubt that's the reason.