Perhaps if you attach a screenshot it will help us understand what you are seeing. What is it that you are trying to download, (please provide the address pasted from the locationbar or from the download page)

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode