Trouble using Firefox for this forum

[Arthurrs]

Anyone else experiencing difficulties with this forum using Firefox 56.0.2 for OS X? All seems ok using Safari, but I keep getting a message like this one in Firefox:

"Not Found
The requested URL /forums/forumdisplay.php was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request."

I can't even log in on Firefox!!!