Introduce our Firefox build to the notion of speed
Enable PGO (profile guided optimisation) build
This change alone significantly helps with the brower benchmarks.
This has been shown to cause unresponsive pages for some Solus users,
and in virtually all cases a reduction in performance. In my own tests
disabling Stylo/Servo helped significantly.
Maniphest Tasks: T4966
Signed-off-by: Ikey Doherty <firstname.lastname@example.org>