ומהו זה שהנביאים אומרים שראו המלאך אש ובעל כנפים. הכל במראה הנבואה ודרך חידה. לומר שאינו גוף ואינו כבד כגופות הכבדים. כמו שנאמר כי ה' אלהיך אש אוכלה הוא ואינו אש אלא משל. וכמו שנאמר עושה מלאכיו רוחות:
"What does it mean when the prophets say they saw an angel of fire or an angel that had wings? Everything is really just part of the [prophet's] vision which is a riddle. These two ideas, [that angels are made of fire, or have wings], tell us that angels have no physical bodies and no weight that comes along with a physical body. This is similar to the verse (Devarim 4:24) that says, 'G-D, your Lord, is a consuming fire.' G-D is clearly not a fire, rather it is a parable. Also, another example is when it says (Psalms 104:4), 'He makes the winds His messengers.' [This is obviously not true either since wind can't deliver messages]."
This idea, that the prophetic visions a prophet experiences are all in the mind, can explain a lot about what the Rambam is saying here in Yisodei Hatorah. When the Bible refers to angels being fire or having wings this is not their true state of being, but rather how the prophet envisions them. This is what is meant in the Rambam when he says that the prophets vision is a riddle. The vision that the prophet receives while he is unconscious has imagery that helps the prophet better understand what G-D is trying to tell him. Therefore, when the angel has a fire like form, or wings, this is coming to aid the prophet in the interpretation of his dream. Also, this sits well with the rest of the Rambam that says any physicality that is given to an angel is completely allegorical and not literal in any way. True, an angel has a Tzura, a purpose, that it comes to teach men, but this has to do with the message that is being sent through the angel. An angel can appear in a fire like form in order to teach a lesson to the prophet and not to mean that angels are physically made out of fire.