Add option for firefox incremental builds
In Tor Browser builds, when we need to rebuild firefox, we do a full firefox rebuild. During the development of firefox patches it would be useful to be able to do incremental rebuilds of firefox.
To do that, we could add a new makefile target for this type of build and an option in rbm.local.conf pointing to the firefox sources directory. We would then mount that directory in the build container and use it instead of the sources from git.
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