The fact that you chose to install it doesn't mean that can never be malware. Some of the extensions being offered for various purposes may appear to be safe while still harbouring some unwanted code that can act as malware or adware. If it's not hiding within Chrome, I would definitively advise you to scan your system for any malware/adware. It may have copied itself across to another browser or somewhere else in your system. These extensions sometimes manage to sneak other bits of software onto your system which may be why you're not seeing the extension itself in the list.
Have a good read through this:
https://malwarefixes.com/remove-free...fox-ie-safari/
As for the additional bits of software that this page recommends you use, I have used Malwarebytes (free) and Sophos in the past and can recommend them. I'm not familiar with the Avast tool, but the company is legit. Just make sure to download these from official sources only, so don't use the links on that page but go directly to the Malwarebytes and Sophos sites and download the software from them.
Apologies if I'm repeating the obvious or bits you've already tried. The behaviour you're describing definitively points towards something on your system that should not be there.