Add a Firefox target to build unpatched Firefox in tor-browser-build

I think we could have a special Firefox target to build Firefox from tor-browser-build.

The main point is that the toolchain update usually doesn't require a (full) rebase.

So, we can do the toolchain update in parallel with the rebases.

Another point is that it will help with potential reproducibility patches to send to Mozilla.

Notice that we don't need a fully working target (e.g., we don't necessarily need to modify the browser project, but it could be appreciated).

Also, I don't think we need Makefile targets for this, we can use rbm incantations.