User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:67.0) Gecko/20100101 Firefox/67.0
Steps to reproduce:
This is not a simple issue to reproduce, but I wanted to create the bug.
Every day I power on my computer (Windows 10), open Firefox, and restore my previous session. In that session I might have a youtube tab with a video I'm in the middle of watching, maybe another video in another tab, and news and technical docs articles in the rest of the tabs. 8-10 tabs open in the window.
At some point in the past couple of weeks, I started noticing my system slowing down. It kept getting worse. I saw that my memory usage had gone over 90%, and that lack of RAM was causing the problem. But I didn't want to go buy more RAM right away.
Yesterday, something happened accidentally that caused me to lose my "previous session" in Firefox, and I had to start with a fresh Firefox session, 1 tab only. Suddenly my memory usage was less than 70% and all my performance problems went away. It was clearly an issue with Firefox. Opening the same tabs again does not reproduce the issue. It seems to take time to build up the excess memory usage. I have 8GB of RAM, so each 10% is 800MB.
Excess memory usage by Firefox, but the memory doesn't show as belonging to Firefox in the Task Manager's Processes tab. Firefox never appears to use more than 1.5GB of memory, and generally less than 800MB. The excess memory usage appears to belong to another process or other processes. I cannot determine the specific processes using this excess memory.
I will be monitoring this going forward, as I like restoring my previous session.