Timer.jsm uses much more memory in content processes than native implementations would.

Not essential for Fission Beta, should get triaged for correct priority with other memshrink work. Below is some context from kmag's conversation on matrix.
kmag: It'll be a few KB. It's unfortunately just a bit messy to implement. None if the existing timer stuff is easily reusable and adding globals to JSM scopes is... not trivial at this point
There is a much quicker short-term version where we'd keep Timer.jsm but have it export a native API like I did for Services.jsm, though. That'd probably take about half the time. Maybe less. So I may do it.
But we'd save a bunch of additional memory from removing the code that imports Timer.jsm. There's a bunch of it, and it adds up quickly. And it's pretty hard to quantify until you actually remove it. I tend to just see huge surprise memory improvements when I do those sorts of refactorings.

This is a memshrink bug, not a Fission bug.