I was reading "An Efficient On-the-Fly Cycle Collection" on the plane today, and in the paper it is suggested that "To save more scanning time, we add an additional stage, the mark live black stage, between the mark stage and the scan stage." Though, in our case this might not be very useful, since it is marking which takes time.

Yeah. I need to do some profiling of the patch with your build to see how it falls out.

Per policy at https://wiki.mozilla.org/Bug_Triage/Projects/Bug_Handling/Bug_Husbandry#Inactive_Bugs. If this bug is not an enhancement request or a bug not present in a supported release of Firefox, then it may be reopened.