Wondered if Firefox was using to much ram/cpu, I checked task manager & saw a 'trillion' items. Don't even know what most of it is ; startups, also. What to keep intact, what to GET rid of ? Wen in IDLE, the load on the CPU is very light. So it MAY be you have some backround taks the is doing some kind of job. Like a vius scan. Or a download. Or a automatic uMicrosoft Windows XP Home Edition date.
What happens if you reboot and discoue from the internet?A trillion is alot...wanna MAYBE provide more details ? ?

BTW it ain't all FireFox's faultMany background processes are related to programs you are running all the time, software and other programs OS runs. BUT unfortunately, there is a lot of malicious files that can use your CPU and appear in the Task Manager as running in the background. I would advise you to fully scan your PC with anti-malware or alternative malware fighting program so you can see which ones are safe and which ones are not.
You can also manually check the location of the concerning file. And then you DETERMINE if it is essential or notHave you compared Fire Fox to other browsers?
Here is a list of bowers that claim to be less hungry for memory.
https://mashtips.com/best-lightweight-browsers-for-windows/
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There are proofs all over the internet about how these giants drain our system memory and the precious battery life and what not, thanks to commercial competency.