(1) is much more acceptable. 'Fireworks' is a display, and although the singular exists, it is seldom used. A noun used as an adjective is normally put into the singular, but here, because of the singular idea of the display, the uncommonness of this particular singular, and the fact that the sound is emitted from the display and not a single firework, the plural is more appropriate.
(2) Grammatically correct but not idiomatic. The subject is 'those', the verb is 'are'.
(3) Correct. 'Some ' is not a plural pronoun, it is a determiner for the singular 'skill'.
(4) Correct and not odd at all. The subject is 'you', and the verb is 'need'.
(5) and (6) have the same meaning: use the phrase 'it is' for declarative sentences like 'it is cold in here'. In contrast, perhaps, to 'is it', which is used in questions: 'is it cold in here?'
(7) is correct; (8) is not. One spark cannot leave a trail, which is a series (of burning particles, in this instance).
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