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Did you also find the Avast antivirus program installed, or was it just the browser extension? And did you already have some active antivirus on your system, with real-time protection enabled?
Avast antivirus was installed as well. I had to remove it from "Add and Remove Programs" and restart.
I don't have another antivirus other than the default Windows Defender that is a part of Windows 10. I'm not sure it was enabled though.
Well with Windows 10, Defender will fully activate itself unless you disable it, or if it automatically detects a 3rd party antivirus solution installed. If it detects another AV, it will go into passive mode. If you uninstall your current AV, Defender will pick up the slack. That's actually a good call on Microsoft's part!
But the installer for Avast had to arrive on your machine bundled with something else. I can't imagine it just appearing out of the ether. In my case earlier, I had Bitdefender running on a Windows 7 machine, and when I updated CCleaner, it installed Avast! There was a known issue where that install was supposed to be optional, but apparently CCleaner missed something in the GUI that did not show ME the opt out. It was probably a rare corner case, but it did happen!
That was long winded, but I must assume something else was updated on your computer that invited Avast aboard. 
I assume so too, I know programs don't get installed from the air 
but I would like to know what it was, and more then that I think the Avast team should find what it was, since things like that will give them a bad rep.
No program, however legitimately installed or updated, should install a different program without asking the user and letting us opt-out.
This is malware-like behavior.
It is malware like behavior on the part of the program that installed Avast. In this case, it isn't Avast doing the installation but,
Avast is being installed via that other program.
One of the reasons to always do custom installs regardless of what you install.
It's also a good Idea to have Unchecky ( https://unchecky.com/ ) running in the background in case you miss something.
I agree, and I always do customs installs.
In this case I'm not aware that I have installed anything, so whatever it was - I believe it was a silent process.
On the face of it, it may be claimed that Avast is "not at fault" if it was silently installed by something else, but when a program is bundled with another it's usually a case of co-operation between the companies, and not out of the "good will" of the installing program.
In any case, until the other program is found and declared, Avast remains the only address to complain to, and the one that suffers from the bad rep.
