Why does FF render html / javascript code one way on my computer, but differently after the code is ftp'ed to my website?

I prepared an index.html file, an associated javascript file, and an associated index.css file. When these were tested on my computer using FF, the code rendered pages as expected. However, after these files were uploaded to my website, they rendered pages differently when accessed with FF, e.g., realignment of text, etc. Why would FF render code one way on my local computer and another way on a new website? == This happened == Every time Firefox opened