Changes to turning off e10s

Gijs Kruitbosch gijskruitbosch at
Mon May 13 11:59:44 UTC 2019

(Cross-post: m-d-platform and firefox-dev)


(If you don't care about turning off e10s, you can stop reading.)

At the end of last week I landed changes in
that may affect how you turn off e10s. The broad aim was to ensure that 
we stop grandfathering users into a non-e10s configuration which they 
should not run on a day-to-day basis, given that it receives little to 
no testing and is less secure.


- Support for the force-enable/force-disable prefs has been removed 
- The browser.tabs.remote.autostart pref being set to false will no 
longer turn off e10s in the following case (ie if **all** of these are 
1. you're running desktop Firefox
2. you're running an 'official' build (MOZ_OFFICIAL build define; 
basically anything other than local builds)
3. you're not running our automated tests (specifically, 

The autostart pref will thus continue to turn e10s on/off for any of the 
- non-desktop-Firefox (comm-central, Fennec, ...)
- for automated tests (so --disable-e10s switches in ./mach will 
continue to work as you expect, and tests that always run with e10s off 
will continue to do so)
- on your own, local, non-MOZ_OFFICIAL builds

Longer-term, I'd like to move those cases away from the pref one way or 
another, and remove the pref altogether.

If you need to disable e10s for manual tests on an official build, you 
can use the (pre-existing) MOZ_FORCE_DISABLE_E10S environment variable.

I expect this to be uncontroversial given the long-ish discussion about 
e10s and automated test support in general in m-d-platform prior to this 
happening. If you spot issues with the implementation, please do file 
follow-up bugs.


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