Since version 45 Firefox Hello (also known as Loop) is separated into an xpi file and can be removed during the installation (or aftewards manually). It is relevant to all OSes including Linux. So it is possible now to remove Hello xpi during installation process and it won't be installed. It would be great to have such USE flag, e.g. "hello". It placed in "/opt/firefox/browser/features/loop@mozilla.org.xpi" In 46 version the same will be done with Pocket.
Mozilla's packaging of firefox-bin is to keep it as verbatim to upstream's package as possible. As such, no I don't think we will add a use flag to conditionally install or uninstall Hello, just like upstream's package doesn't conditionally install it. If this is just a simple matter of not installing an xpi file, I expet you could easily leverage INSTALL_MASK to handle this.
And what about www-client/firefox?
Well as of right now the same INSTALL_MASK (or PKG_INSTALL_MASK if you want it removed prior to binpkg generation) method will handle this for you just fine. For removal of the modules in-ebuild, to be frank i'm not particularily interested either as said restrictions would be one more addition to the list of things I need to check on each new release. But, if there's enough demand for its removal then I'll entertain patches. Even if we go forward with this though, I'd rather not end up having a new use flag for every firefox extension that can be turned into an optional install -- this could get out of hand rather quickly.
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