Ask HN: Will my new company fire me because I don't know their tech stack?

I've been a consultant for over a decade and lately decided to take a job offer at a company that I genuinely like, but the problem is the language they use is one I've never used before. I disclosed that with them when I first applied for the position, and have learned a bit about it, but now I'm afraid that they'd get frustrated with me or fire me if I don't catch up quickly.

Does anyone have a similar experience, or any tips for me?