We've been shipping with CSS containment enabled for several releases now (ever since Firefox 69, via bug 1487493 landing on 70 and then being uplifted to 69).

We might as well remove the about:config pref.

Oh, hmm -- it looks like we have a reftest that depends on preffing this property off, which means it needs to remain preffable for the time being in order for that reftest to continue functioning properly.
The test is:
https://searchfox.org/mozilla-central/rev/fa2df28a49883612bd7af4dacd80cdfedcccd2f6/layout/reftests/bugs/1466008.html
...and here's the reftest.list line where we pref this property off:
https://searchfox.org/mozilla-central/rev/fa2df28a49883612bd7af4dacd80cdfedcccd2f6/layout/reftests/bugs/reftest.list#2054

For now, I'll just spin off a helper-bug to simply change our tests to assume the pref is default-true everywhere (which in practice it has to be at this point) -- i.e. I'll remove all the test lines that manually set it to true. But I'll keep the pref, and I'll leave that one reftest in place (and leave this bug open to track removing the pref once we've got a better solution for that reftest.)

per bug 1466008 comment 13, we can switch to using backdrop-filter (and its pref, layout.css.backdrop-filter.enabled) in that reftest.

I'll plan to take care of this soon, now that helper bug 1626459 has landed.

(Since we've been shipping with it default-enabled for a while now.)

See https://bugzilla.mozilla.org/show_bug.cgi?id=1466008#c9 through #c13 for
notes on the reftest.list change.

This patch's changes were automatically generated by the following command:
./mach devtools-css-db

Depends on D71861

https://treeherder.mozilla.org/#/jobs?repo=try&revision=98e0ea385a431f64e46b7d6c68a6f203f4eb0fe4

https://hg.mozilla.org/mozilla-central/rev/a7d9b81aa753
https://hg.mozilla.org/mozilla-central/rev/b6daad2bedbd

Can confirm that the pref layout.css.contain.enabled was successfully removed in 77.0b8 across platforms (Windows 10 64bit, macOS 10.15.5 beta 3 and Ubuntu 18.04 64bit).