It seems that there are two syllables in "liar" (li·ar) because it is made up of a stem "lie" plus the suffix "ar". But what about "prior" (pri·or)? Does it have something to do with the final "r" as opposed to "fire" which has only one syllable?
"Liar, liar, pants on fire" is a common kid's taunt. Both words get two syllables, to make the line a trochee (long, short, long, short).
The American rock group has a well-known song,
"You know that it would be untrue You know that I would be a liar ..... Come on baby, light my fire"
When they sing it, they make it "fy-er".
Ordinarily, we give "fire" one syllable - or as close as we can.
But keep in mind that English spelling is only a suggestion for the way it sounds.
Like British English "Cholmondeley", pronounced "chum-lee", or "Grosvenor", "grov-ner".