Just adding a data point in case it helps:
On linux/Firefox, I can see a running build page grow from ~40Mb to over 2Gb over the course of an hour or so. the majority of that space, ~1.6Gb is listed under dom/orphan-nodes.
Jenkins 2.190.1
Firefox 60.9.0esr
Phillip Whoriskey, if you could install a recent Nightly build of Firefox 72.0 and install the profiler addon from https://profiler.firefox.com/, you could create a profile which might help to figure out where the leak actually is happening. See the documentation in how to create and upload a profile: https://profiler.firefox.com/docs/#/