Mozilla killed XPCOM because it was actively preventing improving Firefox. In particular, Firefox could finally go multiprocess, and other improvements of the Quantum project are slowly being incorporated.
A lot of people don't view that (or other changes) as improvement. I personally hate multi-process browsers because they eat RAM like nothing else. In my browser of choice I normally run 200-500 active tabs and I stay under ~3 GB of ram usage. With a multi-process browser that'd be impossible.
I don't know if you've forgotten, but under that old use case (hundreds of tabs in one process), Firefox would bog down further and further as background tabs stole more and more main thread time, eventually only a restart of the browser would restore it to usability.
In 2019, I still use hundreds of tabs - and Firefox handles it with grace. RAM is there to be used, and this is the perfect use for it.