In 4E and 5E there seems to be the requirement of a Wall of Fire to be resting on a solid surface. But in 3.5 I did not find such a rule. So: can you create a Wall of Fire in midair?
There's nothing in the 3.5 rules to prevent you from placing a wall of fire in midair. Nothing in the spell description, or the specific rules for Evocation spells, or the general rules of magic requires that you place a a wall of fire on a solid surface.
Some other wall spells have the specific requirement to anchor to a surface, but wall of fire doesn't.
Interestingly, in another very good answer regarding prismatic wall in the Pathfinder ruleset, Hey I Can Chan notes that objects summoned with Conjuration spells cannot appear in open air, and this rule also appears in D&D 3.5. Since wall of fire is Evocation, this limitation doesn't apply.