There has been a virus named firefox.exe It is basically a miner bot, that hogs your resources. While I am not certain about this, it definitely is a possibility, as Firefox usually has very little resource usage overall. You could try doing a thorough PC cleanup to confirm or deny suspicions on that front.

RKill to kill any malicious processes -> Check programs for recently installed ones that might be malicious -> Run a full scan with anti-virus software of your choice -> Scan with Malwarebytes, Hitman Pro, and AdwCleaner -> Use CCleaner to clean up registry and cached files -> Use Firefox cleanup tool -> Check if it works properly now. Let me know if this helps!

Hi   !

Would you please take a look at the article :

Firefox uses too much memory (RAM) - How to fix

This could also be caused by Multi-process (e10s)

If you are using e10s, you could turn it off as follows :

Type in the address bar about:config   (press Enter) (promise to be careful, if asked)

Type and search for the preferences :

browser.tabs.remote.autostart and set it's value to false   (default)

browser.tabs.remote.autostart.2 and set it's value to false

Note: the exact name of the preference may vary, but it will start with browser.tabs.remote.autostart