Firefox (as well as most new browsers) cache content fairly aggressively. They do this to speed up the website experience and to make sites appear "snappy".
For developers, this often leads to the head-scratching exclamation, "I thought I fixed that! Why is it still not working?!?
Here is how to clear the browser cache in Firefox (version 57 and newer) for a single website:
Click the menu in the upper right corner.
Select "History" and then "Show All History" (it's at the very bottom).
In the "Search History" field, enter the domain name of the offending site.
Right-click the domain name in the list and select "Forget About This Site".
Refresh.
That should do it but sometimes it seems to revert back to the previous content. (We don't know why.)