“Firework”, singular, is outside of The Hundred Thousand.  OED tells us that in its meaning as “a pyrotechnic display” although the plural “fireworks” only is used now, the singular used to be used.  Most instances in The Hobbit are of “fireworks” except one.  The uninflected form could be the older form for the meaning just cited – or could be the form indicating other, obsolete meanings of “firework” (work done in fire, a furnace, and others).  Of course… Tolkien uses it in paragraph 01.091 adjectivally, to describe the glare of the blue light on Gandalf’s staff.  OED admits of no adjectival uses, except as the first element in some hyphenated word phrases.  The word we know is tweaked so very gently off the rails – we can take nothing for granted, yet we do not know consciously that we have been alerted.  Absolutely elegant.

• 01.017 such particularly excellent fireworks!
• 01.018 You seem to remember my fireworks kindly,
• 01.092 in its firework glare
• 06.030 (even the hobbit had never forgotten the magic fireworks
• 07.083 I would have given them more than fireworks!’
• 14.013 No fireworks you ever imagined equalled the sights that night.

“ˈfire-work | ˈfirework, n.” OED Online. Oxford University Press, March 2015. Web. 5 June 2015.