As with many lines of employment it is incredibly important for firefighters to dress for success to maximize their potential. In many lines of work this simply means dressing the part to look good but for firemen it actually is a matter of life and death in many cases so it is absolutely crucial. There are tons of different pieces of gear for firemen to wear on the job and each and every one of them serves an important purpose, otherwise they would not bother carrying around the excess fweight while working.
The first important piece of a fireman's gear is jacket and pants combination that provide the most basic level of protection when things get hot. These are important because they are literally the most significant line of defense between a firefighter and the heat from the fire they might encounter. When it comes to fireman clothing there really is not anything more important than these two pieces, you will absolutely never see a fireman working a fire without them, you can be sure of that. Protection is really the basic principal of almost everything a fireman would wear too so it really should not be too surprising to hear that this is so important.
The next more important piece of gear for most firemen is what they wear on top of their heads. The combination of a fireman's helmet and mask are yet another critical safety measure that all firemen use in basically every case. The helmet obviously serves to protect the wearer from heavy objects or debris but the mask serves a much more important purpose. In most cases a fireman's mask serves as both an eye protection system as well as a breathing support system, without it firemen absolutely couldn't survive to do their job. It is pretty obvious that the helmet and mask combination are also a very important part of fireman clothing, so odds are you will never see a fireman working without them nearby.
Obviously clothing is a big safety factor for firemen so it shouldn't be a big surprise that these 4 items are the most important pieces. Each fireman or firefighting unit has their own equipment and clothing policies so don't be surprised if there are other pieces of equipment you do or do not see next time you see a fireman.