Looks like you're infected with some malware. Maybe cryptomining?.Reset Firefox to default settings (google that if you don't know how), and if that doesn't help enough delete the Firefox profile in your appdata\roaming and appdata\local folders and start over. Yes, appdata is an hidden folder, so you should tell File Explorer to show it.
Scan with MBAM (free download) and with your antivirus to see if those find more you don't want on your PC.
I had big problem with google chrome because it was working in background eating around 50% of cpu without even opening it or appearing in anywhere except the task manger,
i uninstalled it and installed firefox, now firefox makes the same thing.
as you can see in the image < https://ibb.co/e7rkin > that annoying background thread of firefox takes about 28% of my cpu and goes up to 30% causing cpu to boost to 3.5GH wheres the opened firefox itself takes 5% and goes down to 1%.
not to mention this firefox background program causes my laptop to heat up.
my lap specs
core i 7700HQ
any ideas!

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