DU4 Bug Reports

First:

Check to see if Firefox is up to date:

1. Click the "hamburger" icon (the three horizonal lines) at the top right of your browser window.

2) Click "Help" then "About Firefox"

3) A window will pop up which will tell you if your browser is up to date -- if it isn't, you should take the option to update it.

If that doesn't work:

Cache and cookies

Sometimes clearing these can help. There are simple instructions here, note that on there is further reading on the right side of the page if you have any questions:

https://www.whatismybrowser.com/guides/clear-cache-history-cookies/firefox

Might be worth a try...