For future reference (in case I forget) and for anyone who might ever experience this particular problem: I've been wondering for the past several days why my Firefox browser wasn't allowing me to scroll down web pages with my spacebar the way it usually allows me to. I thought there might be something wrong with my keyboard, but the spacebar worked for inserting spaces. Then I realized I could see whether it was Firefox or something else by checking Internet Explorer. It worked fine for scrolling in IE. That meant it had to be some setting in Firefox or some setting elsewhere that would somehow affect Firefox but not IE. The former seemed much more likely.
So I looked through every option in the various menus Firefox has, and only two seemed to have to do with scrolling. Neither changed anything. I even looked through the help file to see if these two functions were supposed to do something that they weren't doing. No, they did things that were unrelated to my problem. In the process I did notice something else strange. It listed Page Up and Page Dn as moving to the top and bottom of a web page. They didn't do that. They moved to the end of whatever line I was on. That should have been a clue. A little later I noticed that when I clicked on parts of a page with no text the browser was acting as if I had clicked on a line with text. That was clue #2, and I reminded me of a menu option that I had dismissed as irrelevant, which turned out to be exactly the problem.
So here is the solution: Under Options, select the Advanced tab. Accessibility Options will appear. Under Accessibility Options, one choice says "Allow text to be selected with the keyboard". That option was checked. I unchecked it, and the problem was solved. Now I can use my spacebar to scroll down web pages.