I have been a very happy Firefox user for several years but recently (with 104) I noticed how it uses massive amounts of CPU when being idle. About 60-120% CPU are not uncommon. In the task manager there is no task with heavy CPU load. It seems to get somewhat better over time, but it is really significant, making my Macs quite loud with just an idle Firefox.
You can start with your description and end with "I'd like to help, but I don't have the knowledge how to debug this further. I'm happy to provide more information if you can guide me."
The first thing you'll be pointed at is likely https://profiler.firefox.com/docs/#/./guide-getting-started so it may be a good idea to start with capturing the "idle" profile for a while and submitting that with your initial report. Check the "Share a profile" section.
