Searching about:config for e10s, I see I'm in the "disqualified-control" "e10s.rollout.cohort". Looking further, I see "extensions.e10sBlockedByAddons" and "extensions.e10sBlocksEnabling" are both true.
So, one or more add-ons are presumably incompatible with the feature, but even if I uninstalled those (which ones?) I might still be in Mozilla's control group, and there's no clear way to override that.