Looks like geckoview is using this: /Users/dave/mozilla/build/trunk/obj-browser-dbg-full/_virtualenvs
Bug 1531475 Comment 4 Edit History
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Looks like geckoview is using this: https://searchfox.org/mozilla-central/source/mobile/android/geckoview/src/main/java/org/mozilla/gecko/GeckoThread.java#298