Nah, browsers are sandboxed to absolute shit it is such a pain in the ass to make an extension just to do a phishing attack or to buy the ownership of one to introduce malicious code.
At most an extension with really broad permissions like read/write contents of any page (a fact that is made obvious upon installation) can replace a link to take you to a phishing page to harvest creds, but thanks to SSL and HTTPS it won’t even work without fifty some odd warnings