If your mother is on SSI (Supplemental Security Income) it is not taxable. If she is on Social Security Disability, then some of the income may be taxable IF she has additional income. If she has no taxable income, then she will meet the income portion of the rules to be your dependent. The next step is to determine the cost of her support. If say it costs $30,000 per year for housing, food, medical, clothing, transportation, etc., and you pay more than half of this amount, then she will be your dependent, and you may claim Head of Household status. I suspect that if you provide this information to FSB, it will clear this up.
Please let me know if I can help you further.
I have one more questions related to Head of Household qualification. If my mom does not live with me but lives in the government subsidized apartment, can I still claim myself as a head of household?