I should add that I'm talking about "Cached Web Content". Not the Application Cache.

Hello

1. Click the menu button and choose Preferences.
2. Select the Advanced panel.
3. Click on the Network tab.
4. In the Cached Web Content section, click Clear Now.
5. Close the about:preferences page. Any changes you've made will automatically be saved.

For details follow this - How to clear the Firefox cache

Thanks, but you have to read more than my title. I specifically said that this was *exactly* what I did, and it didn't work.

Maybe that page is not updating properly. If you close the Preferences page and re-open it again does it recalculate to a lower figure?

It's also possible that disk space was not yielded back to the OS right away. To see how many items are stored in cache, you could try another page. Type or paste about:cache in the address bar and press Enter or Return to load it. The disk category is the most interesting usually. Does it still show a large number of entries or has that come down near zero?

Well, for a completely different reason I did a run of MalwareBytes on my machine. Nice utility! Now everything seems to work. Though I seem to have to click "Clear Now" twice to make it happen. So I have to assume that this problem was because of some malware on the system. Aaargh!

But your other diagnostic strategies are helpful. Thanks!