I've had this problem before in the past with Firefox on the computer I use as a home server and for downloads.

How many tabs do you have open in the browser? I found the more tabs the greater the memory usage. I was also getting 90% CPU usage.

I did fix the problem eventually but can't remember what it was as it was some time ago. Possibly it was a virus checker constantly checking the browsers open connections. Kaspersky I think because I ended up ditching that due to it slowing down the system and becoming very annoying.

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