Typically after less than 30 minutes of browsing, and fewer than ~12 total tabs I will get into a situation where Pale Moon consumes 100% of any one of the PCU cores at any given time. It is doing right now as I type this. The application still performs decently well, but my CPU fans don't love it.
(I don't seem to have this problem on the 4 other Windows 7 machines I run, so I'm reporting this in the Linux section.)
I will then open a new tab to about:blank and close all of the other tabs, one-by-one, slowly trying to see which one is to blame. The CPU usage never drops, even once I've closed all of the original tabs (but leaving the single blank tab open) and left things for a few hours.
Once I close the application, after ~30 seconds the Pale Moon process relinquishes the CPU and memory, closing seemingly properly.
Starting the browser again will have me in this situation again withing 30-60 minutes. More intensive web sites like Facebook cause this to happen sooner, but are not required.
I'm sorry I don't have better steps to reproduce the problem. If I can narrow it down, I will.
I have Pale Moon remember which tabs I have open upon closing, and it restores them when I re-launch the browser. That is a new change for me, and shouldn't have anything to do with this problem. I actually turned on this setting because of this problem. It is saving me time, now that I am restarting PM so often.
I know I've left some details out, so please let me know what else I can provide. I will be happy to oblige.
Operating system: Linux Mint 20.1 (64-bit)
Browser version: Pale Moon 29.0.1 (64-bit)
Problem URL: Various, nothing specific.
Installed extensions: Also listed in hidden below. (GitHub Web Components Polyfill | uBlock Origin)
Installed plugins: (about:plugins): "No installed plugins found"