The study investigates the creation of flame-retardant flexible polyurethane foam (PU) made from castor oil (CO) in varying densities. This naturally flammable foam is treated using a dip coating process with concentrated hydrochloric acid (HCl) and sodium silicate to form a protective silica hydrogel layer, thereby enhancing its fire resistance. The characterization of the foam was done through thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy. Evaluation through the underwriter’s laboratory (UL) 94 ratings and burning rate measurements demonstrated significant enhancements. Intriguingly, while non-coated foam samples received V-2 ratings, the coated counterparts exhibited markedly higher ratings of V-1 and V-0, highlighting their superior flame resistance. Specimen (S3) demonstrates greater performance, exhibiting a burning rate of 6.72 mm/min in vertical conditions and 6.16 mm/min in horizontal conditions. Notably, this sample displays a notable difference of 0.56 mm/min between its vertical and horizontal burning rates, indicating its consistent and commendable fire resistance across varying orientations. Findings revealed a direct link between foam density, burning rate, and increased flame retardancy. This innovative approach holds immense promise for an array of applications across consumer and commercial sectors, including but not limited to furniture, carpet underlay, bedding, automotive interiors, and packaging. This research lays the groundwork for creating safer and more durable materials for a range of everyday products by utilizing sustainable resources and advanced flame-retardant techniques.