The Firemaking cape is pretty useless currently. There are already so many different light-providing items, including wearables and those that don't detonate gas pockets. On top of that, the most important dark caves can be permanently illuminated with Fires of Eternal Light anyway.
There are also already a number of very accessible capes that provide warmth in Wintertodt's lair, and since the HP cape provides health regen, and four warm items gives the maximum bonus, there's basically no real reason to use the Firemaking cape over any other.
Put together, this means that max/main accounts literally have no reason to ever equip the cape.
One way to give the Firemaking cape some real use in this game would be to give it immunity to cold damage—both the direct damage that Wintertodt applies to the player and the passive area effects in cold areas that drain the player's stats, such as the entrance to the God Wars Dungeon.
Wintertodt would still be able to hit the player with the AoE snowfall and the brazier exploding, but for players that have already maxed the relevant skill, it would be nice to not be constantly interrupted out of fletching and feeding the brazier. I think it's safe to say they deserve it, if they're still doing Wintertodt after 99 Firemaking.
And as for the area effects, it only makes sense that a cape that is both a source of light and warmth, and a mark of mastery over fire, should be able to protect the player the same way the Fire of Unseasonal Warmth does. Firemaking is already a neglected skill as is, and fighting off the cold is one of the few real perks to actually training the skill. Let's expand on that value, shall we?
