The rationale for this is the same as for bug #140745: we would like to
the WebExtension API, and DOM bindings, via GObject Introspection.
the GNOME stack, and itself uses the Mozilla SpiderMonkey JS engine.
WebKit already ships JSC, and having both JS engines running in the same
process does not sound like a great idea. The alternative could be to use
Seed  instead, which uses JSC, but it seems pretty much unmaintained
(some GNU/Linux distributions are still shipping it, so it could be a
workable solution, dunno).