Molten metal-resistant fabric is very important for workers working in foundries like steel and aluminium. In this study, three samples of jute–cotton union compositions with different weaves have been manufactured and treated with durable organo-phosphorous-based flame-retardant finishing. The differential scanning calorimetric (DSC) study carried out on untreated and flame retardant (FR)-treated fabric samples showed a lower thermal decomposition temperature in the case of FR-treated fabric. The TGA study also revealed a higher residue remained at 400 °C of FR-treated fabric. The FR-treated fabric samples were studied for their molten metal splash resistance (iron and aluminium metals) before and after 5 wash cycles as per ISO 11612 standards. Out of three samples of different weaves—plain, twill, and mock leno—only the twill weave 2/2 twill fabric passed the E3 level, while the other two passed the E2 level. In the case of the molten metal splash (aluminium) test, all samples passed only the D1 level. These FR-treated samples also passed the minimum criteria of limited flame spread, convective, and radiant heat transmissions as per ISO 11612 standards.