Firlefanz doesn’t have a literal translation, but the closest it gets to one is probably “frills” or “frippery.” If you hear someone say the word Firlefanz, it is mostly used in a disapproving way.
If you're looking for a synonym, Schabernack, Unsinn, Tand, Ramsch are all words for something unnecessary – whether in the form of thoughts or objects.
The word dates back to the 14th century – back then, though, it had a more positive meaning: It was used to describe a funny and quite quick dance.
That is until the grammarian Johann Christoph Adelung gave it a new meaning in his “Grammatical and Critical Dictionary of the High-German Dialect” in 1796: He used it to describe a goofy farce or an unfunny, quick thought.
Ever since then, Firlefanz has been used to describe goofy behaviour, gibberish or unnecessary things.
Bei deinem Firlefanz mache ich nicht mehr mit.
I will not participate in your frippery anymore.
Worüber auch immer er da redet, ist doch auch nur Firlefanz.
Whatever he talks about is nothing but gibberish.
Ich habe gestern beim Aufräumen sehr viel Firlefanz gefunden und weggeworfen.
When I was cleaning up yesterday, I found loads of unnecessary stuff and threw it away.