What is the difference between $AC^2$ and $FO (\log \log n)$ complexity classes?
The functions $\log \log n$ and $\log^2 n = (\log n)^2$ are quite different. The latter grows much faster than the former: $\log \log n = o(\log^2 n)$. This shows that FOLL is a subset of AC2 (and even of the smaller class AC1).

Uniformity is a different aspect in which the two classes differ: FOLL is a class of uniform circuits, whereas the AC hierarchy captures non-uniform circuits. As such, even AC0 circuits can compute uncomputable functions, something that FOLL cannot do.