Thanks for that, Douglas.
Start by simply trying another browser, such as Edge or Chrome. (I'm not recommending either of those browsers but simply suggesting those that you're likely to already have on your computer). If, as I suspect, the problem disappears, you need to go back to Firefox and then . . .
1. Clear the cache:
https://support.mozilla.org/en-US/kb/how-clear-firefox-cache
(I suggest restarting Firefox after you've done so).
That might well fix the problem but, if not, . . .
2. Reset Firefox to its 'out of the box' state:
Firstly, note down the names of any extensions that you want to retain, as a reset will remove them and you'll need to reinstall them afterwards. (Click the three lines, at the top right. Select Add-ons and Themes > Extensions to see which extensions you've currently got installed).
Then go to Three Lines > Help > More Troubleshooting Information > Refresh Firefox (at the top right) to carry out the reset.