Start using LTO for firefox project
There are a number of platforms where Mozilla is using LTO in their builds. We should switch to that if possible by setting the respective MOZ_LTO env variable.
This is the parent ticket.
We might want to test this thoroughly as there are probably reproducibility issues involved (glandium mentioned the other day that Mozilla's macOS builds are not reproducible anymore which is not the case for us; the best explanation he could come up with was that LTO is causing that).
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