I would need to check soures to be sure however Silverfire does appear more powerful then Spellfire. OTOH the sphere/wall is not a single barrier of protection , rather it is a series of protection. Further spell level tends to be a factor.
"Small beings can have small wisdom," the dragon said. "And small wise beings are better than small fools. Listen: Wisdom is caring for afterwards." "Caring for afterwards ...? Ker repeated this without understanding. "After action, afterwards," the dragon said. "Choose the afterwards first, then the action. Fools choose action first." "Judgement" copyright 2003 by Elizabeth Moon
I would need to check soures to be sure however Silverfire does appear more powerful then Spellfire. OTOH the sphere/wall is not a single barrier of protection , rather it is a series of protection. Further spell level tends to be a factor.
Thing is Wall of Force is a lower level spell than Prismatic Sphere and Spellfire was able to wipe a Prismatic Sphere away, yet not a Wall of Force.
Prismatic Sphere is an offensive barrier, while Wall of Force is nothing more than a pure barrier.
The only way that I know of that Spellfire can destroy a Wall of Force is for the wielder to go up and absorb the wall directly.