Virus protection on iMac 2013

You’re five macOS versions back, and a dozen years. Six versions, counting the macOS beta.

A whole lot has changed in macOS itself and its security in recent years, beyond the security updates.

Scrounge yourself a less-old Mac. Preferably, one with better performance.

Or as suggested above, install Linux or one of the BSD UNIX distributions, and use that.

Rent a Mac from MacStadium or from one of the other providers, where you need macOS.

An add-on security app isn’t going to fix this. Firefox ESR might help, but you’re already as far back as that goes.

Bluestefan57 wrote:

I use the iMac for internet use and managing my groups etc. Should I install an antivirus software.

The answer to that question is no.

I am loathed to install any 3rd party software because of the knowledge they can be ineffective. But as I am not getting any security patches etc I am worried.

While macOS "Big Sur" will no longer receive any more updates, it continues to receive XProtect updates, as do systems even older than it. The last one was issued a few days ago.

Mac app security enhancements - Apple Support

Effective defenses against malware and other threats - Apple Community

The best advice we can give you is to let that machine go and buy something newer that's actually supported.

If you can't afford that, consider installing a linux build on it instead of running macOS. The version of macOS you're running is as far as that machine can go and hasn't been supported in years.

Catalina is enough to run the current version of the Firefox Web browser. You may want to use that instead of, or in addition to, Catalina's old version of Safari.