See http://kb.mozillazine.org/Flash#Performance_or_display_issues_with_certain_Flash_videos which sounds like the problem your mother is experiencing.

Thanks but as I said in my question I have already disabled the hardware acceleration for flash and updated the video drivers.

Yes, I realize that, but there's more info on that site which describes other causes.

I have already read the page several times and it doesn't have anything that helps.

I too have read that page over and over and tested my flash by going to the adobe website and nothing works. I have the latest version of everything, yet nothing I do makes the flash movie work in Firefox. I am a web designer and have never had this problem until recently. The flash movie works in IE, Chrome, Safari and even in Netscape, but not in FF. I've asked no less than a dozen people to test it for me and it doesn't work for them in FF either. The player is visible and the image titles change as if the photos were showing, but the window is blank in FF. It also does not loop continuously, as it should in FF. It does in all the other browsers. The images I've attached are from the test example I was making for my client. As you can see, the left one shows the images just fine in Chrome (and is the same in other browsers. The right side image from FF, clearly shows there is nothing showing within the flash window on that page.

URL of that web page?

http://lori-h.com/MsDenver/JBpaintingsTest1.html (BTW, I know the links on the left don't work. They aren't supposed to be working at this point).

Is there an email address to contact FF/Mozilla directly about this?

That is caused by hotlink protection on the server.

Firefox doesn't pass the referrer to the Flash plugin and the server refuses to send the image files because of that.

http://lori-h.com/MsDenver/slides/p_0001.jpg
GET /MsDenver/slides/p_0001.jpg HTTP/1.1
Host: lori-h.com
User-Agent: Mozilla/5.0 (X11; Linux i686; rv:10.0) Gecko/20100101 Firefox/10.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive

HTTP/1.1 403 Forbidden
Date: Tue, 07 Feb 2012 23:05:01 GMT
Server: Apache
Content-Length: 347
Keep-Alive: timeout=15, max=98
Connection: Keep-Alive
Content-Type: text/html; charset=iso-8859-1

Ok, I'll contact my server, but why then, does it work on other sites I've created using the exact same server? I'm just curious. This only started happening recently. I checked the other sites and they are still operating just fine in FF. I also compared the permissions and they are identical. As I said, I will contact my server, but I don't understand why one doesn't work when all the others do. Thanks!