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Give this one a try on Chrome:
https://chrome.google.com/webstore/detail/get-cookiestxt/bgaddhkoddajcdgocldbbfl...
I know for sure it works as many people are using it. If it doesn't work for you, then there is something else going on on your end. Maybe some kind of ad or tracking blocker? Not sure!
@ trustycakes and everyone else, the "Export cookies.txt" addon on Firefox works well but you have to rename the given cookie file "r18.com-cookies.txt" as "r18.com_cookies.txt" (otherwise the program will not recognize it)