Linux users don't bother with AV. Malware writers don't write viruses and trojans for it because the market share is small and the reward is low. And Linux has more than good security.
Sounds like an extension was installed somehow or other that hijacked your homepage. An extension would survive a bleachbit cleaning. It would be nice if you could retrace your steps the night before this happened. Later...
Given the sparse information to go on, the posts are mostly, and understandably, speculative in nature. I didn't see anything at all that appeared to be FUD.
Well I have not faced anymore ransomewhere or malwares again since I took a few steps to secure the linux mint I have. Hopefully it was a once in a life time happening.