Clearing your web browser's cache might not be such a familiar process for you, but it is often the solution to many problems you'll face while using Moodle and the internet in general.
What is a cache?
Simply put, it's basically your web browser's memory used to speed up processes. Sometimes you need to wipe it out for things to function correctly. This is a harmless and simple process that will not cause any harm to your computer even if the cache wasn't the root of your problem.
If that does not work, try the instructions below.
Clearing Cache in Mozilla Firefox
First, click the Menu button in the upper right-hand corner as seen in the image below.
Next click on the History icon as seen in the image below.
Next select Clear recent history as seen in the image below.
Finally, check only Cache, choose how far back you want to clear (i.e. Last Hour, Last Two Hours, etc.) and select Clear Now at the bottom of the window.