The Answer is... inconsistent or rather manyfold:
Edition Wars
Dragonlance used several (A)D&D Rulesets in its history as an RPG supplement, and thus has the standard edition inconsistencies:
Starting with AD&D around 1984, it was probably one of the first supplements to adopt (prototype?) rules of AD&D 2nd until 1993. Using these "Rules in Development" might be the reason for some inconsistencies, but it is hard to tack down each change.
At the point Legend of Huma was published - 1988 - AD&D 2nd was officially used for all the products, yet the publication history did keep up adding more editions later:
A module of 3 standalone books followed in the 1990s, which didn't demanded other books, and to which I couldn't find to which rule version it belongs (Inn of the Last Home).
Finally the newer iterations from 2003 onwards used D&D 3.5 and at the same time (2006-2008, Age of Mortals Campaign) were published using d20 rules. The changes from AD&D 2nd to D&D 3.5 and d20 are huge at times and again can be the source for extra inconsistencies.
YET: I couldn't find any indication that silver Dragons ever breathed fire in the Monster Manuals (see below!)
Campaign Setting
But keep in mind that Dragonlance provides a full standalone campaign setting, just like Blackmoor Forgotten Realms or Eberron. As a full campaign setting it often provides also own rules and at times monster statistics, which overwrite the standard (A)D&D rules as they were provided in the core books. Thus another possible source of inconsistencies.
However, there never was a Dragonlance specific bestiary, and I couldn't track down any change to the breath weapons in the modules (AL2 & DLA1) I had at hand.
Development History
Atop that, Dragonlance was first invented around 1981/82 on the game table of Tracy and Laura Hickman1, with the idea to develop AD&D Modules from the adventures of that group. This means, that the first prototype documents for Dragonlance were pretty much House rules. Yet, they planned to base the first dozen modules on the Monster Manual available in 1981, so most likely the 1977 one.
This book lists Dragons starting on Page 29. It brings us different Dragon breaths... including the one for silver dragons (Draco Nobilis Argentum) on page 34:
SPECIAL ATTACKS: Breath weapons + possible spell use
[...]
When in combat a silver dragon is able to attack with claw and fang or use either of its two breath weapons - a cone of frost (cold) 8" long with a 3" base, or a cloud of paralyzing gas 5" long by 4" wide by 2" high which will take immediate effect upon all within it unless they save versus dragon breath.
In the AD&D 2nd Monstrous Manual the following is written over the Silver Dragon:
Breath Weapon/Special Abilities: A silver dragon has two breath weapons: a cone of cold 80' long, 5' wide at the dragon's mouth, and 30' wide at the end or a cloud of paralyzation gas 50' long, 40' wide, and 20' high. Creatures caught in the cold are allowed a save versus breath weapon for half damage. [...]
Novels
But the project also pulled in Margaret Weis, who subsequently was responsible for the novel series (published starting 1984). In the novel series, rules that were established in contemporary AD&D or the own Dragonlance supplements often were bent and broken to fit the narrative. At other points, scenes were exaggerated for impact.2 As such, the Dragonlance novels hold some inconsistencies to the Dragonlance modules, simply by having to provide aditional info or paint pictures in the mind of a reader and not give some Leeway for a GM to describe things in his way.
Conclusion
Legend of Huma was one of the novels in the Dragonlance setting. As such, while based on the Dragonlance modules and AD&D Rules of its times, it holds the freedom of the author with the ruleset, and diverged from the published and established rules:
The silver dragon of AD&D and AD&D 2nd Edition was gifted with Frost and Poison Gas and the ability to cast spells with a chance of 75%3. Fire breath was not in its powerset.
It is not impossible though, that he might be able to cast some similary looking cone of fire using his mouth as point of origin - but that would be handled as the spell. Possible ones might be Cone of Flames or Scorching Ray
Notes
1 - Word of mouth has it, that Tracy Hickman, Harold Johnson, Larry Elmore, Carl Smith and Jeff Grubb were the players and that Gary Gygax gave his ok to the idea to develop the adventures centered around one Dragon each.
2 - One random example of the novel diverging from the Adventures from the Chronicle novels: Dragons of Autumn Twilight. Around page 400-450, Matafleur was described as almost blind on one eye and fully on the other and in very poor condition healthwise - heay scars, ripped wings etc, suggesting to a genre savy reader she might have lowered Str and Con stats in contrast to her category. Yet she managed to kill Ember a much fitter dragon in a rage filled suicide attack by slamming herself and the target into a mountainside and burying both in rubble. The novel makes this a very huge deal, describing it like some heroic feat
In contrast AL2 - Dragons of Flame has her stats. They are not very different to Ember but for being not listed as huge (might be an error), having a slightly lessened bite damage and having no listed magic - both are "Ancient" and otherwise almost identical. Her description though features one eye as "clouded and possibly blind". In the adventure, this is much less an uphill battle of Matafleur/Flamestrike, yet she gets a nicely scripted battle Ember/Pyros. The end result is not given in the same way here, but death of both parties seem implied.
3 - The corresponding article on spellcasting Silver dragons in the AD&D Monster Manual:
Silver dragons who are able to use magic are able to employ a maximum of ten spells. At each of the first two ages they gain two 1st and 2nd level spells respectively. At each age thereafter they gain an additional spell - 3rd level at subadult and young adult, 4th level at adult and old, and 5th level if very old and ancient. Thus, a very young silver dragon of this ability would know two 1st level spells, a young one would know two 1st and 2nd level spells, etc. Furthermore, 25% of magic-using silver dragons have books of spells, so they may select spells for use rather than knowing only a limited number per level.
