It's two words. Both of them are nouns. Fire is a noun that modifies another noun, truck. Same as saying dump truck, delivery truck, garbage truck, tank truck, etc.
A compound word is a word that is made up of two or more other words (e.g. dragonfly is made up of two words, dragon and fly).
New words formed by fusing together parts of existing words are known as blends. They used to be called portmanteau words, but this term isn't used as much nowadays (e.g. brunch, guesstimate, motel, fanzine, and shopaholic).
Blends are not strictly the same as compounds, which are made by joining whole words together, rather than parts of words.
See some examples of common compound words from EnchantedLearning.com.
And here 25 favorite portmanteau words from DailyWritingTips.com.