Automatic safe boot - Password not reckognized

Hi everyone,

I own a MacBook, bought in 2008 with Leopard as an OS. It is the balck model, if this helps.

While I was browsing, Firefox started behaving strangely. It was taking forever to minimize windows and could not use the top right function to view all applications/windows open. I restarted the mac but when it booted up again it did not do it properly. For 10 minutes I had the greyish screen with the loading sign underneath the apple logo, then eventually it showed me the safe boot mode, asking for my password. When I enetered it, it did not accept it (login window shaked at each attempt). I tried to log in from the CD, but again asked the password and did not accept the one I put in, even though I know it was the right one!

Now I can't use the mac and can't access safe boot either. Would you know any workaround solution for this, like a way to reset the password?
Have you got any clue on why the mac decided all in a sudden to boot up through the safe boot mode?

Thank you very much for your help!