Content processes (web)
Media plugin (Adobe, Cisco...)
NPAPI plugin (flash)
Default Firefox Sandbox
Seccomp (kernel) on Linux
Extra Firefox Sandbox
SELinux (sandbox -X -t sandbox_web_t firefox)
Wayland
Faster (no drawing)
Secure (no access to other windows, root window, screen...)
...
much more!
Quantum
build next-generation web engine
Gecko + components from Servo
more parallelization and GPU offloading
CSS, Render, Compositor, DOM and Flow
but Firefox will drop something too...
Test Pilot experiments
what and how is used
potential features (if successful)
regular add-ons (if less successful)