sandy_locke: Mer/Nemo or Mer/PA are just good environments to test stuff if you are building Mozilla tree with Qt, and Mer SDK is rather easy to set up and it's up to date tools wise and maintained by the community. It's also shared by all Mer based projects, so you don't need to adjust it too much for each target (most probably armv7hl build will work on both Nemo and PlasmaActive, though you can set up both targets in Mer SDK).

If you need to work with Harmattan or Fermantle - you can probably do that their way, but it's not necessary, unless you specifically want to build for them. (We are talking about building Mozilla tree itself, not about just writing the QML UI for embedlite).