Quantum I think was too incremental and conservative, trying to integrate Servo into Gecko. They’ve ended up making a desktop browser which is as good as Chrome, but not good enough to demand attention. Mobile, where parallelism should be most beneficial, has lagged behind, and parallel layout, which is where the real prize is, has disappeared over the horizon. Servo has been relegated to WebVR experiments which are…
The next thing seems to be Fission, which is the continuation of the e10s (electrolysis) content process model into full/more Site Isolation. ( https://wiki.mozilla.org/Project_Fission - but it seems hard as fuck: https://mail.mozilla.org/pipermail/firefox-dev/2018-July/006... nowadays they seem to be stripping out the old Gecko XUL stuff and replacing it with something newer and better: https://mail.mozilla.org/pipermail/firefox-dev/2018-November... )