Hi, thanks for the report, and sorry it didn't work for you. You were right: in Firefox the add-on was broken. It was originally built for Chrome, and it asked Firefox to clear two data types that Firefox doesn't support. Instead of skipping those, Firefox rejected the whole request, so nothing was removed.

This is fixed in version 0.1.25. Cookies, history, cache, local storage, IndexedDB and service workers are cleared again.

One note: Firefox doesn't let add-ons clear every kind of site storage (for example the Cache Storage that websites keep for service workers). So the "Cookies and Site Data" size in Ctrl+Shift+Del should drop a lot, but it may not reach exactly zero.

If you give it another try, I'd appreciate your feedback!