In addition to trying another browser, which will tell you more about whether this is a browser issue or a web page coding issue, I would also suggest clearing your browser cache in Firefox (or even all your browsers).
Cache corruption can cause all kinds of weird problems and clearing it often makes them disappear.
If you are a CCleaner user it includes the option to clear cache for each browser installed on your computer, or you can do a web search on the browser name and the phrase "clear cache" you'll find scads of step-by-step instruction sets for any browser you can name, though including the browser version along with the name these days is a good idea, too, since the exact steps have changed over time and version.
Brian - Windows 10 Home, 64-Bit, Version 1803, Build 17134
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back