If you click the Clear Data button again -- without actually clearing data -- is it listed as cache?

To view an index of what is in the web content cache, type or paste about:cache in the address bar and press Enter/Return to load it, then click into the disk cache. You will usually find that Firefox has retrieved content for the built-in Firefox Home/new tab page, but there might be some other content retrieved in the background.

I concluded that the source of the problem must have been elsewhere; the repair shop had installed the iteration i was using... Glad the resolve came by uninstalling and reinstalling mozilla app., the cache data now/then indicated 0 bytes.