Medical eligibility items have the following ratings: Yes, No, or Maybe. Humidifiers are given a "Maybe" rating by some administrators. This means a health care professional must sign a letter stating the humidifier is necessary for a particular physical condition. The letter must include the diagnosis. A detailed plan must be filed outlining use and duration of treatment.
Other companies list humidifiers as eligible, so it's necessary to check the individual manager. (see References 2 and 3)
The FSA is held and administered by a third party which determines if the request for reimbursement qualifies under your plan. This can be an insurance company or other entity that handles benefit programs. If payment is denied there are appeal procedures. If that fails, your rights include bringing a civil action.
FSA plans are contracted on a calendar tax-year basis. For a humidifier to qualify, you must have enough money left in your plan for that year. Conversely, if there is money left in your plan at the end of the year, the balance is forfeited and a new plan starts on the following January 1.
Congress, along with the IRS, controls the rules, so they could change at any time.