I swear this hasn’t always been like this:
Post a comment
Click the edit button
Look below the grey editing box (that contains light buttons for ‘save’, etc.
The Edit and Quote buttons are still there. So you can click ‘edit’ to edit a post you’re already editing… 
That’s actually a quick edit screen you’re seeing. It’s new since the upgrade, and allows you to edit your text without loading a full edit page with all the fancy-schmancy formatting buttons. If you need the full edit page, just click on “Go Advanced” in the quick edit screen.
Hitting the edit button again gives you the same result as the “Go Advanced” option. The quote button does exactly what you’d expect.