I prefer to use the arrow keys to scroll while reading web pages. Every now and then, Firefox (version 184.108.40.206) stops responding to those keys. The Home and End keys are ignored as well I notice. Today I stumbled upon a unexpected, but practicable, solution (about 3/4 of the way down the page): choose Options from the Tools menu and then click the OK button.
What's mind-boggling is not so much the solution, but that it was discovered at all.