The only way I can get Firefox to say "File not found: Firefox can't find file at" is to use a file: URL like: "file:///tmp/does-not-exist". So the error you mention seems to indicate that you are using a file: URL to try to access a file on a remote computer. Perhaps the file that works is a file that you ALSO have on your workstation as well as the server, and the file that doesn't exist does not exist on the server? You can verify that it is actually requesting the file from the server by looking at the server logs (as I'll mention below).
When Firefox reports "File not found", what is the URL in the URL bar at the top? Is it "file:" or "http:"? If you go into a shell window or file browser, does a file with either of the names you are trying to access (the .php file that works or the file that doesn't) exist on your workstation?
The URL you are using should look like: http://serverhostname.example.com/filename
Here are some other general Apache troubleshooting ideas using the logs on the server, which may provide more information and verify if you are actually hitting the server or not.
The first thing to do is look in the Apache error logs, typically in "/var/log/httpd/error_log". Try accessing the file that gives the error, then look in the error_log and "/var/log/httpd/access_log and see what they contain related to that request. Note that if you have made configuration changes to Apache and are serving files out of a different directory, your error/access logs may be in a different location.
Normally, if it is a permissions problem on the server, Firefox would respond "You don't have permission to access on this server". The error_log would say "(13)Permission denied". In this case you may just need to "chmod 644 " so that it is readable by the web server. If that doesn't work, sometimes it's SELinux blocking the access. You can test that by running "setenforce 0" and trying to access the file again. If it now succeeds, the problem is related to SELinux. Use "setenforce 1" when you are done to re-enable SELinux.
If it can't match the URL to a file on the file-system, firefox would normally respond with "Not Found: The requested URL was not found on this server", and the error_log would say "File does not exist". This would normally be caused by a naming or typo issue, verify what the logs say against the file name in the file-system.