False. We "hit if off straight away" refers to getting on well straight away. For instance: Mr X: "I met this lovely girl the other day. We hit it off straight away!" Mr Y: "I guess you'll be seeing each other again then!" It means that Mr. X met a lovely girl and they got on straight away/instantly. On the other hand, "get on like a house on fire" is less of an immediate thing. It's more of an ongoing or 'permanent' thing. Mr X: "James and I get on like a house on fire! We've been best friends since we were 5 and I'm going to be best man at his wedding!" It means that James and I really get on well. We're such good friends and we have been for a long time. -- Occasionally you will hear "got on like a house on fire" in the past tense and it does imply an 'immediate' getting to know someone in the right context. Mr X: "I met this lovely girl the other day. We got on like a house on fire!" Here it suggests an 'immediate'-ish (approximate) friendship... But even used this way, it implies that you were "getting on like a house on fire" for some time and that you at least had a full conversation. I hope this helps and take care!

Just like to add that we don't usually say (or at least I've never said "get on like a house fire"). I have said We really hit it off, though.