firefox running slow

My desktop installation (v 151 on W.10) has suddenly started to hog the CPU. With a single tab open it is using 22/25% of the CPU. I closed a couple of extension and made no change.

With only the Help tab open - Task manager shows 1.0% But, with a single tab (BBC news page) it jumps to the 22/25%. I attach a screen shot.

The number in brackets increments with each tab - so why does it show (12) when I only have the Help page open?

I have used Mozilla as my primary browser since the days of Netscape!! I have never had this problem before but have had to shut it down.

Thank You. SA (in UK)

my firefox is running slow