This bug is not currently tracked.
This metabug will track the start-up improvement work we aim to get done for 2019 Q1.
To be clear, our goal is to improve cold start time of Firefox Desktop, with an emphasis on Windows. We'll be benchmarking using the Quantum Reference hardware.
According to Standard8, metabugs in Firefox :: General usually get set to P3 (even though, oddly, the underlying bugs here will likely be higher priority).
Q1 is done - closing the books on this one.