a11y current has the following dependency on the e10s build flag as identified by Felipe:
The code I was referring to is the following function: http://mxr.mozilla.org/mozilla-central/source/accessible/src/msaa/nsWinUtils.cpp#186 (nsWinUtils::IsWindowEmulationFor)
If a flag is still needed you can change the flag to a pref check for browser.tabs.remote on runtime.
Created attachment 576456 [details] [diff] [review]
inbound land https://hg.mozilla.org/integration/mozilla-inbound/rev/29f5a947d06a