www/firefox: temporarily disable OMTC to avoid crashes

OMTC on X11 platforms uses BasicCompositor unless GL layers are
also enabled. Change the default value of its pref in about:config.
For now, bundled cairo is left as is pending more testing.

No upstream bug as I cannot reproduce reliably to get the stacktrace(s)
with a clean profile.

PR: 202174
Reported by: many
Tested by: Martin Birgmeier, Andrey Fesenko, Konstantin Belousov
MFH: 2015Q3
X-MFH-With: r393690