Fire bombs (more properly referred to as fire grenades, since the common
usage of the former term is incendiary, rather than fire-suppressing!)
as we know them, probably originated in the mid 19th century.  As early
as 1820, large carboys of fire extinguishing mixtures were brought to
fires to be propelled through the hose at the flame.  The first patent
for a bona fide fire grenade seems to have been 1863.  Fire grenades were
essentially bulbous bottles (complete with long necks) which held fire
suppressing chemicals like carbon tet.  There were numerous patents for
similar grenades through the end of the century.
 
The grenades had a great range in size, anywhere from three to
eighteen inches high, partially resulting from differing neck lengths, I
suppose.  They were generally mold blown, often with embossed maker's
marks.  The glass ranges in color from clear to yellow to amber,
aqua, and various shades of blue green and purple or amethyst.
Generally, they will have a base, and many are ribbed, footed,
whatever.  The simple clear spheres seem to date to the late 19th
century and the 20th.  Such globes were suspended in a wire rack
(of, say, six grenades) in a glavanized steel box or "fire
extinguisher."  In this case, the whole box was supposed to be
thrown at the base of the flames where the heat would melt the
wire and release the globes to fall and break.  One
manufactures of such products were International Fire
Equipment of West New Brighton, Staten Island, New York.
 
The Friendship Firehouse Museum here in Alexandria, VA has several
examples of 19th-century fire grenades.  I will forward any info I can
obtain from them.
 
Otherwise, the best source of information is probably bottle collectors
and their literature and organizations.  A fraction of them specialize in
the collection of fire grenades.  The source for most of the information
I have included here is William C. Ketchum's "A Treasury of American
Bottles" (New York: Boobs-Merrill, 1975).
 
In conclusion, however, I would guess that any thin, clear glass sherds
are, as others have suggested, lamp chimney sherds.  There were, however,
several other types of clear, globular bottles, including "witch balls,"
testing bottles, fisherman's net floats, Christmas tree balls, and target
bottles for target practice.  These are certainly rarer and a less
likely find unless their manufacture or sale was occurring in the
vicinity of the archaeological site.
 
Tim Dennee
Alexandria Archaeology
Alexandria, VA