If using the 'header' function for the downloading of files, especially if you're passing the filename as a variable, remember to surround the filename with double quotes, otherwise you'll have problems in Firefox as soon as there's a space in the filename.
So instead of typing:
<?php
header("Content-Disposition: attachment; filename=" . basename($filename));
?>
you should type:
<?php
header("Content-Disposition: attachment; filename=\"" . basename($filename) . "\"");
?>
If you don't do this then when the user clicks on the link for a file named "Example file with spaces.txt", then Firefox's Save As dialog box will give it the name "Example", and it will have no extension.
See the page called "Filenames_with_spaces_are_truncated_upon_download" at
http://kb.mozillazine.org/ for more information. (Sorry, the site won't let me post such a long link...)