This is actually my preferred behaviour but I don't know what is the correct behaviour. I'll check older versions and get a better idea of what they did.
Bug 1634272 Comment 8 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Although I'm unsure what the browser "should" do, earlier versions focus the address bar in all cases, and that seems consistent so I think it should probably do that.