If your school uses Canvas, yes. It works on every instructure.com address right away. If your school uses its own address (like canvas.yourschool.edu), click the FixCanvas icon while you're on Canvas and hit "Enable on this site".

Nope. Install it, open Canvas, done. FixCanvas works on top of the Canvas login you already have in Chrome, so there's no password to type and nothing to sign up for.

FixCanvas only changes how Canvas looks in your own browser. When you submit or reply, it uses Canvas's own features, so to your professors everything looks normal.

Anything about your classes: when stuff is due, late policies, what's on the midterm, what your grade is, what your prof announced, what's in this week's slides. Answers come with numbered links to the exact Canvas page or file.

Any time. "Canvas view" in the sidebar switches back with one click, and removing the extension puts everything back exactly how it was.