I was waiting to see whether there were going to be any more guesses.
The answer was d) Curtain.
And, yes, nogods, I noticed the similarity in the words for window in other languages when I was trying to come up with alternate guesses. I'd never thought of that before, but it struck me that there might be a common root in there, since curtains and windows go together, as it were. It had also occurred to me that the first three letters (fir), might be related to vir, which seemed as though it might have something to do with glass as well, but I was probably thinking of vitreous, which is a bit of a stretch. Anyway, it's always fun to think about things like that.
I remember first getting fascinated with the relationships between words in other languages when I was about 8 or so, and someone had taught me the German words to "Silent Night"...and a couple of days later, I was walking around in my bathrobe, which is "szlafrok" in Polish, and it suddenly hit me that the first part, "szlaf", was pronounced just like the "schlaf" in the carol, which meant sleep, and "frok" was just like the English (and probably German) word for a dress. Anyway, that epiphany was way exciting to my 8 year old mind!