The growing demand for sustainable solutions for flame retardants (FR’s) in polyester (PET) fibers raises questions about the efficacy and environmental impacts of these products. This theoretical review study included the effective technical parameters, application processes, tests, various physical, chemical, thermal, mechanical, and surface morphology changes of applied FR treatment were technically examined and interpreted. The main purpose of this theoretical review study was to examine the technical aspects of various physical, chemical, thermal, mechanical, and surface morphology changes in polyester (PET), and polyester (PET)/cotton (CO) blended fabric structures, especially in their FR finishing processes, and to guide future technical studies. FR applications have been generally used in military, textile, automotive, and metallic industries. The type of chemical used, concentration (by volume % c), viscosity (Pa.s), molecular weight (Da), pH, temperature (C), time (minutes, or hours), pressure (Pa), low free surface energy, thickness (mm), and environmental conditions (especially are relative humidity (rH), and atmospheric pressure (Po) etc.) of the coating applications have been effective factors on FR applications. The dip-coating, pad-dry cure, sol-gel, layer-by-layer (LbL), and plasma-grafting application methods have been applied for FR applications. The washing (especially are the soap, alcohol, or distilled water), drying, and fixing processes have been applied as post-processes, respectively. LOI test has also been the most important test, too. It must be over 21% ratio. Triazine, formaldehyde, melamine, halogen, phosphorus compound chemicals, ZnB, and silica (Si) nanoparticles have been commonly used as non-sustainable and non-eco-friendly chemicals. Polydopamine (PDA), chitosan (CHI), casein, protein, enzyme, DOPO, APA, β-CD, and boric acid have been commonly used as sustainable and eco-friendly chemicals. They are also FR effectiveness, low smoke, and toxicity, biocompatibility, and cost-efficiency chemicals. In conclusion, the alkaline, or plasma grafting as pre-treatment processes should be applied before applying FR applications. The optimization for FR application should be varied between 8% and 20% (by volume) for the concentration, between 60 °C and 80 °C for the temperature, between 0.5 hours and 3 hours for the time, 7 for pH, 1:10 for flotte ratio with using pad-dry cure, or sol-gel processes. The between 80 °C and 160 °C for temperature for between 3 minutes and 5 minutes for time with distilled water, or ethanol chemicals, which are washing chemicals as drying, and fixing processes, respectively. As the FR concentration increases by volume (% c), mass loss, and burning time increase, and the burning rate decreases. It has also self-charring behavior, too.