'whereis' failed to work... any reason?

i am new to Mac.
i run 'whereis' program before and apart from the program path, it showed me the other directories the program is also using... (i use whereis firefox to check the directories) but lately it suddenly doesn't work... just nothing except going back to the prompt.
someone said the application (firefox) is not under the PATH, i checked and understood, but just don't get it why whereis work before and now don't.
i tried whereis whereis and it replied /usr/bin/whereis
how to make it back to before?

thanks.

If you type in you will see the list of directories whereis will look. If Firefox is not in one of those directories (which it shouldn't be), it won't show up. I can't explain why it showed up in the first place unless you put it in one of the standard binary directories (which should avoid doing as there is no benefit to you for doing so).

If you want more info on whereis, look at the man page: