I have tested to confirm the Waterfox-oriented CAA addon is usable with firefox v52.9 ESR.
note: >>>>> I cannot vouch for (nor provide support for) the CAA addon nor any addons retrieved from the archive server.
FYI, each “xpi” file is a zipfile (renamed to .xpi so that the browser treats it as an installable addon).
You can, and I encourage, right-click “SaveAs” rather than clicking to install individual addons.
Once you have a local copy (which you can backup for safekeeping), you can
1) rename to .zip
2) extract and inspect/edit the contents (at a minimum, I edit “install.rdf” and blank the “update/hompage” URL string)
3) repack into a zip, rename to .xpi, and paste the file:/// URL of the local copy into browser addressbar to initiate installation