Update: A quick search for Firefox add-ons led me to Reading Ruler, which looks like it may well be the solution I'm looking for — will let you know how it works out...
I've noticed that I tend to scan ahead when reading, and sometimes skip over words without thinking. Holding a bookmark horizontally across the page and moving down as I proceed through each paragraph helps, when I'm reading a physical book. Now I wonder if there's something I can do to achieve the same thing on web pages.
Zooming in to make the body text take up more of the screen is a partial solution, as is Reader View. And having Firefox narrate the text to me makes the text scroll as it is spoken, with highlighting to show each word, but I still have a tendency to jump ahead using the cursor keys.
I'm not sure what has led to this state of affairs. A symptom of my autism, perhaps? I don't think it's dyslexia, as I can read the words just fine. It's just that my brain always seems to be in a rush when I'm reading.