This addon seems to be obsolete now that Pihole v6 made its official release. The API has been rewritten for Pihole v6, so this addon no longer works. Also, the source repository on github has not seen an update for 6 years. I recommend switching to PiHole Browser Extension by badsgahhl, which has been updated to support Pihole v6 and is actively maintained.

with the latest version of pi-hole, it doesn't work anymore

This was an amazing plugin but it doesn't appear to work any longer and doesn't seem to be getting updated, have found "Switch for PiHole by badsgahhl" to be a good replacement.

This was an amazing extension! Both here on Firefox and on Chrome. However, over the last day or so, it simply stopped working. The icon is a bare with no status displayed as it no longer connects to the Pi-Hole server. Just a test button now. Both browsers too. :(

Unusable.
Plugin doesn't connect to Pi-Hole.
No reaction from the devoloper.