I have an iMac (Core 2 Duo) running Thunderbird 115.8.1 on Mac OS X 10.13 High Sierra. Thunderbird works fine when I'm actively using it. The problem arises when I close the window and switch to a different app. If I let Thunderbird run in the background for a while, the processor gauge (iStat Menus) will indicate that Thunderbird is using more than 100%, typically about 120-130% cpu usage. As soon as I switch back to Thunderbird (click on it in the Dock), the processor usage drops way down to normal (about 2%) and will stay that way as long as Thunderbird stays in the foreground as the active app. If I close the window and switch to the Finder or Firefox, Thunderbird will eventually start overusing the CPU again. It doesn't happen immediately... probably about 20-30 minutes of being idle in the background. I've got Thunderbird set up to check one Gmail account. Nothing else. I used to have a different email account, but removed it. Checking the logs in Console hasn't been helpful thus far. High Sierra seems to flood the logs with unrelated information. I've noticed this issue for several revisions. I kept hoping that the next update would fix things. No such luck as of 115.8.1. Anyone have ideas how to fix this? Edit 2024-03-14: I changed the subject to indicate that the problem occurs when the window is closed. It has nothing to do with being in the background vs. foreground.