What figure of speech is this: "the arguing sixth floor window"

I think it is a case of personification:

a figure of speech in which a thing – an idea or an animal – is given human attributes.

The non-human objects are portrayed in such a way that we feel they have the ability to act like human being

I think synecdoche is most correct answer here, although it is a surprising example. Synecdoche is not always strictly about a relationship of a part to a whole. A classic example is that an athlete is a jockstrap, or "jock". The jockstrap is not part of the athlete. Another example is where a container is used to refer to its contents, e.g. "He drank the keg". I think this example is similar to that. It would be more common to say something like "The sixth floor was arguing", but your example works the same way.

Definitely a figure of speech, it may be more specifically defined, but I feel almost certainly is a form of irony.

The first OED definition of irony is as follows. Take particular note of the parts I have highlighted.

As a mass noun. The expression of one's meaning by using language that normally signifies the opposite, typically for humorous or emphatic effect; esp. (in earlier use) the use of approbatory language to imply condemnation or contempt (cf. sarcasm n.). In later use also more generally: a manner, style, or attitude suggestive of the use of this kind of expression. Cf. ironia n. The meaning in quot. a1657 is obscure. See also Romantic irony n. at romantic adj. and n. Compounds

The precise application of the term has varied over time and remains the subject of much discussion. Irony is first recorded as a rhetorical figure used in sentences and (later) extended pieces of writing having a particular tone and intent. In 20th-cent. criticism application of irony has expanded to encompass non-verbal expression in fields such as art and music where it denotes a distancing from and playful engagement with what has come before. For a fuller discussion see E. N. Hutchens ‘The Identification of Irony’ in ELH (1960) 27 352-63 and N. Knox The Word 'Irony' & its Context, 1500–1755 (1961).

If anything— this is an example (albeit, perhaps a sloppy one)— of metonymy.

My reason for going with metonymy instead of synecdoche is that with the former, the substituted word is merely something associated with it, while with the latter, a part represents the whole or vice-versa.