My backward and forward arrows are missing when I download Firefox 4.0

My brother was at my home this past weekend and he downloaded Firefox 4.0 to his Windows Vista laptop and the download worked perfectly. When he downloaded Firefox 4.0 to my Windows Vista desktop, the download is missing the backward and forward arrows are missing and I find it very difficult to use the Firefox application. My brother removed the program and redownloaded the program at least 4 more times and every time the program downloaded with no backward or forward arrows. Both applications were downloaded from the same Firefox 4.0 website. How can this happen and what can be done to correct it?