@Lucia: I thought it's clear that I'm asking no question :)

Does Firefox 14 still exist (this must be quite old)? -- I think the recent version is 46.0. -- Anyway I can reproduce the bug also in Firefox 46.0.

Maybe. -- Though it can also be a useful pointer when closed as a duplicate (and not deleted), I think.

I think you cannot undo a flag, but a single spam flag has no consequences for the flagged post (it takes 6 spam flags to delete).

@LasseRempe-Gillen: As to the second sentence of your comment: 3 people have already chosen "leave closed" in the reopen votes review queue, so the review is finished, see here.

How to properly cite a post authored by a user who has deleted their account? -- If the author's name is replaced by "user#####", it is in general not possible to find out who the author is.

quid has quit -- quidquid agis, prudenter agas et respice finem? -- Making so immense contributions to the site without even telling who they are needs a quite special personality, so perhaps sudden disappearance is part of the performance and was already planned years ago when creating the account and choosing the name. -- But who knows, maybe all this is nothing but a quiddity!

Just two points: firstly, questions with answers with score $>0$ cannot be deleted unilaterally by the OP; secondly, nothing is really deleted -- i.e. deleted questions and answers remain visible for users with at least 10000 points.

Opinion-based questions tend to lead to lengthy discussions which many, if not most, people don't find worth reading afterwards -- at least not as much as to-the-point questions and answers. A question / answer site like MathOverflow (and to some extent also the other SE sites) aims at gathering information which is useful for future readers, and not only for the participants of a present discussion.