When more add-ons become available, I don't think that a lot of users would have a problem with such a browsing shift - for small and medium time periods at least.

I do think that anything beyond a medium browsing session, or a session where some type of productivity is being tried would tax the user a bit much - either to the point of redesigning the browser (engine, interface, available extensions, etc.) or just the user moving to a larger-screened (size) mobile device.

*side note: I wonder why Fennec's extensions aren't cross compatible with microB