cache png
(17.34 KB, 730x175)

cache-empty png
(17.56 KB, 738x182)

after-visiting-kohl-i... png
(17.29 KB, 720x169)

Make an experiment. Document it and post results.
1. open up your FF, close every tabs, leave only one empty tab
2. goto Preferences
3. goto "Cookies and Site Data" in "Privacy & Security"
4. Make screenshot like picrel #1, give it a name "cache-base.png"
5. Click "Clear Data...", check every option and "Clear"
6. Make another screenshot of the part I picreled, name it "cache-base-empty.png"
7. open new tab, leave the preferences/privacy tab open
8. in the new tab goto Kohl/int
9. go back to the preferences/privacy tab and refresh it
10. make new screenshot of the cache, name it "cache01.png"
11. clear cache as previously
12. make new screenshot of the cache, name it "cache01-cleared.png"
13. you can stop, or repeat steps 8-12, with modifying step 8: refresh Kohl/int instead of opening it, in another tab, and incrementing the number in the screenshots' filenames by one with every iteration, of this loop

Compare results.

I haven't followed exactly the procedure described above, but my three screenshots:
#1 this would be my "cache-base", after regular use I dunno when cleared cache, I rarely use FF
#2 cleared cache, 0 bytes
#3 after the browser loaded Kohl/int front page, I haven't opened up a thread, note this number will and should differ every time you repeat steps 8-12 according what and how many images are on the front page at individual loads
I haven't made a screenshot of that, but I successfully cleared the cache to 0 byte after visiting Kohl/int.