One of the biggest advantages of Anglo-Saxon English—and one of the reasons for its popularity throughout the world—is its propensity for single-syllable words (I, you, he; come, go, fly; good, bad; go, see, look; pop, corn, etc.)—which means that compound words made of two simple terms are often no more than two or three syllables (e.g. airport, background, bedroom, cupboard, football, flyover, highway, takeover, etc.).
Unfortunately, the attraction of such brevity is often lost on the wordsmiths of the venerable Academy of the Hebrew Language, who sometimes think only in terms of semantic accuracy when conceiving of Hebrew equivalents for foreign words. The result is often unwieldy constructions, such as brerat-meḥdal for “default”, or yesumon for “app”, or—my favourite—tetzugah maqdimah for “preview”. Not surprisingly, in those cases people often prefer to carry on using the English word.
Generally, you’ll find that the invented Hebrew words that do catch on are the ones that are shorter or at least no longer than their English counterparts—e.g. monit (taxi), meqarer (refrigerator), maḥshev (computer).
In the case of popcorn, the initial proposal was to call it pitzputzei-tiras (five syllables, no less). Then (upon realising that no one was going to use a name that’s almost longer than it takes to eat the stuff), they proposed the shorter, catchier, tirsah—but it hasn’t caught on as yet, possibly because it’s missing the onomatopoeic sound of “pop”, or crunchiness (which pitzputzei-tiras at least had).
Consequently, I would propose something like tirpatz—but I wouldn’t bet against popkoren going away any time soon…