Anti-Virus For old Desktop Imacs

Hi All

I have a still vigorous custom made made (with usable dvd drive) IMac 27' Inches High Sierra Version 10.13.6 from the Mid 2011's. I have being using Firefox as my browser and (knock on wood) my protection with the Firefox extensions has been amazing. As you well know, Firefox gets updates. However, I was wondering if I need to buy antivirus protection for this great desktop. And if so, what would be a good one to buy from the internet. Thanks in advance for your time and courtesy.

p.s. Does the motto apply: if is not broken don't fix it?