“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.

Tolkien uses “merry” in its present uses – amusing; jolly – and in a number of its obsolete ones, as musically pleasing, or enjoyable, or boisterous.  I imagine that drawing the line between shades of meaning for the OED must be a complex and very artistic profession.  “Merry” does not appear in Chapters 4, 5, 6, 10, 11, 12, 14, 16.  It’s distributed evenly on both sides of Chapter 10.  I notice that most of the second half “merries” begin here:

[18.019] ‘There is more in you of good than you know, child of the kindly West. Some courage and some wisdom, blended in measure.  If more of us valued food and cheer and song above hoarded gold, it would be a merrier world. But sad or merry, I must leave it now. Farewell!’

I think this is what Richard Blackwelder meant by passages “of great beauty”.  I regard “merry” as a healing word: it’s used by Thorin to heal the rift between himself and Bilbo; it fills the farewells as Bilbo leaves behind his war experience; elves welcome him back to the west with “Merry.”

• 01.051 Quite a merry gathering!
• 01.122 They built the merry town of Dale there
• 02.027 So after that the party went along very merrily,
• 02.028 in merry tales,
• 03.023 talking merrily with them.
• 03.026 and they sang a merry song as the party went across.
• 03.029 and some elves have over merry tongues.
• 03.040 and its merry bells,
• 07.070 in my merry men,
• 08.048 and there was a merry singing,
• 08.058 and laughing merrily.
• 09.025 and laugh merrily.
• 09.039 They had left a merry feast
• 09.044 and make merry
• 09.046 and became mighty merry all of a sudden.
• 09.052 the elves being very merry
• 09.064 and there was a merry racket down by the river.
• 13.034 in merry mood,
• 15.043 and grew merry;
• 17.059 that should have lived yet long ages merrily
• 17.061 and the merry elves.
• 18.019 it would be a merrier world.
• 18.019 But sad or merry,
• 18.042 for now the northern world would be merrier
• 18.044 Merry be the greenwood,
• 18.044 and merry be all your folk!’
• 18.051 and merry there;
• 19.012 Merry is May-time, and merry our meeting.
• 19.012 Merry is May-time, and merry our meeting.
• 19.015 Well, Merry People!’
• 19.018 and he had many a merry jest
• 19.020 Merry is May-time!’
• 19.030 And through the merry flowers of June,

Update 2015.07.09: we discovered further on that “merry” may very well be a not-quite-perfect elf-detector.

Blackwelder, Richard E. Tolkien Phraseology: A Companion to A Tolkien Thesaurus Tolkien Archives Fund, Marquette University, 1990. Print.

“merry, adj.” OED Online. Oxford University Press, March 2015. Web. 11 May 2015.

Our current use of the word is for our near relations, not for the sense of ancestral stock as it is generally used in The Hobbit.  “Kin” comes up more in the book as we get closer to the war for gold, to reasons to divide into “us” and “them”.

• 03.035 my kin.
• 09.018 from their kinsfolk
• 12.072 and where are his kin that dare seek revenge?
• 15.021 that you would send messengers to our kin
• 15.040 The king of friend and kin has need.
• 15.051 you would have paid to our kindred,
• 17.033 We are hastening to our kinsmen
• 17.057 O my kinsfolk!’

“kin, n.1.” OED Online. Oxford University Press, March 2015. Web. 11 May 2015.