What is going wrong with Firefox graphics?

Please see attached file. For a few weeks now I've had problems with graphics - but only when using Firefox. I've had Microsoft Support check for any technical problems but they assured me there was no fault with the graphics. The fact this doesn't happen with any other programme either seems to narrow the field down to the Firefox browser. Sometimes text is garbled or there will be a box (as in the photograph) of either white or containing text/picture from another part of the screen. I would be grateful for advice on if and how this can be fixed. Thank you.