Job 41:19-21 ESV
"Out of his mouth go flaming torches; sparks of fire leap forth. 20Out of his nostrils comes forth smoke, as from a boiling pot and burning rushes, 21His breath kindles coals, and a flame comes forth from his mouth".
These Job verses are describing "Leviathan" and your question is about "dragons" in Isaiah. If leviathan and dragon are synonyms for large "fiery" animals then before we dismiss dragons as "could not possibly be fire breathing", we can afford to think of similar animals. e.g. leviathan.
Today we have words like "Plesiosaur"; "Triceratops"; "Dilophosaurus" and "Brachiosaurus" to describe various animals. These names do not appear in the Bible so perhaps when looking at these animals people used a word which the KJB translates as "behemoth" or "leviathan" or "dragon".
KJB Job 40:15-17 "behemoth.. he moveth his tail like a cedar". Cedar are enormous trees which start wide and get narrower at the top, but elephants tails are short and are not tapered. [Brachiosaurus had a tail like a cedar tree].
But in NIV Study Bible footnote "behemoth" is "possibly a hippopotamus or elephant".
My point so far then is to see that the naming of large animals in the O.T. and understanding those names today, is open to all sorts of problems. e.g. Did Job have one name for an animal and Isaiah another name for the same animal?
But we can say that Job saw a fire breathing animal. "His breath sets coals ablaze". Job 41:21 ESV.
In his book "The Great Dinosaur mystery" Ken Ham puts forward many ideas on this subject, including on how animals might produce fire. He mentions a book in the British Museum called the "The Anglo-Saxon Chronicles" which
"records encounters people had with dragons, and many of the descriptions fit well-known dinosaurs". [from Ken Ham].