The Firefox team has been very aggressive about performance lately. Each extension you have installed increases the startup time, so that's probably why they baked it in.
So it improves the performance for a very small number of Firefox users at the expense of the performance of all of the other Firefox users who never would have had the extension installed in the first place?
If it is changed to an addon, it will still be installed by default. It will just be easier to remove. That means most users will take the performance hit because most people don't mess with the defaults.
