how playfire work about achievements i unlock before signin up ?

It won't count, you're outta luck for those.

You only get credits for the achievements you get for the games they currently have rewards for.

well they say it won't work with previous unlocked achievments, but on many occasions they still rewarded me for old achievements. Well their achievment scanning routine is kinda horrible, so it is probably not intended ;)

If you haven't played the game since you joined playfire, launching the game will still give you achievement money in my experience. If you got the achievement after you joined playfire, you are out of luck.