I got this problem back in 2015. Whatever i did, clean installs, messing around in about:config, trying to fiddle with stuff on the Windows side, every time i opened a new tab my CPU usage would rocket up to 100% and hang there for a minute before cooling down. Never figured out what the problem was.
I switched to Pale Moon, and it has issues of its own (being an obscure branch browser, and them switching to a different code-base called Gecko has created some useragent problems), but it's always run smooth as butter.
Odd thing is i've been using firefox at work consistently since then and haven't had a single problem with it, so this must be an issue that develops in isolated circumstances.