This pronunciation can be broken down into two parts. First, the word "first" is pronounced as /fɜːrst/. The "f" sound is pronounced as a voiceless labiodental fricative, similar to the sound made when blowing air through slightly parted lips. The "ɜːr" sound is a stressed vowel sound, similar to the "er" sound in words like "her" or "sir". The "s" sound is pronounced as a voiceless alveolar fricative, similar to the sound made when producing a snake-like hiss. Finally, the "t" sound is pronounced as a voiceless alveolar plosive, similar to the sound made when producing the word "top" or "tap".
Second, the word "floor" is pronounced as /flɔr/. The "f" sound is pronounced as a voiceless labiodental fricative, similar to the previous explanation. The "l" sound is pronounced as a voiced alveolar lateral approximant, where the tongue makes contact with the alveolar ridge and air flows around the sides. Finally, the "ɔr" sound is a stressed vowel sound, similar to the "or" sound in words like "bore" or "more".
So, overall, the correct pronunciation of "firstfloor" is /fɜːrst flɔr/.