What was Thomas Jefferson the inventor of the light bulb afraid of?

Thomas Jefferson did not invent the light bulb. Humphrey Davy invented the first incandescent light bulb and the first arc lamp in 1802. The "Thomas" part may be confusing you a bit with the confused attribution of Thomas Edison as the inventor of the incandescent lamp. Edison is not the inventor either; he simply made it commercially feasible.

Now, as far as Jefferson's fears, those were likely taken to the grave with him and anything else is a subject of speculation.