If its a contract with unlimited employment time, like most contracts in Germany are, he cant fire you anyway, at least not without a valid reason, which could include: stealing, not doing your job, always coming late when there are meetings, stuff like that. But you can only properly be fired for that, if you do it multiple times (depending on the severity), you will get written warnings beforehand.

Another reason is mass layoffs, but they have to have a valid reason too, which doesnt include "oops we hired too many people last year".

So under normal circumstances you will not get fired in Germany, its a different story if you only have a 2 year contract for example, then you might not get a renewal, but a company cannot indefinetly rehire you with a limited contract, they will have to give you an unlimited one, if they want to keep you for longer.