Take heed. This might be quite important and relevant. I've written a short tutorial showing how to troubleshoot, isolate and work around the issue of slowness in Firefox 65 on close, resulting in high CPU usage for extended period of time, including step-by-step component search, 32-bit vs 64-bit options, safe mode and EMET mitigations, the use of previous version and ESR build, accessibility services, and more.