Run a System Restore to a time prior to the changes you made. If that fails or cannot be done, note the error message (if any) there as well as exactly what's displayed when you try to launch programs such as Firefox. That one in particular should not be dependent upon any other installed software, so it's a good place to start looking for what happened.
I came home from my summervacation and decided to remove unused programs. So i uninstalled tons of programs. This made made my computer, in some way, **** up. There are some programs which i cant open, f.ex. iTunes, Firefox and all the standart games on microsoft such as Chess. Some of the programs are gone in some how, and i didnt accedently removed them aswell (why should i remove all my Adobe programs?). And others such as iTunes just says: "iTunes stopped working".
I've tryed to uninstall iTunes and install it again, but it didnt change a thing. I've also tryed to install some of the programs i uninstalled to see, if it removed some nesesary files - didnt work either.