Do you use an add-on to view the books? If so, the add-on might be maintaining that folder (Firefox doesn't use your main profile folder to cache documents). Since these files might track your place in the book, or could have some license-related purpose, if they are stored there by an add-on, you might want to check with the publisher of the add-on for advice.

Actually, could you clarify something? There are two different file paths containing \Mozilla\Firefox\Profiles. One is under Local Settings, and one is not. The one under Local Settings also contains your cache. If this is in that area, it may well be temporary storage that you can clear out. Could you check on that?

Many thanks for your prompt reply. I've used an add-on - "epubreader" so Ill contact epubread.com and see what they say

Per your second reply, the Local folder only has bits'n'pieces in it.

I found the rogue are using a programme - "Spacemonger" which gives a graphical display of where on a drive memory is being used

I'll post another reply when I get a reply from epubread.com

regards Bernard