This add-on was great until recently, when it stopped working. I keep getting the same error message others have mentioned: "Sorry, something's not quite right here. Please check that you are logged into Pinterest and try again." I am logged in! Ok, so I logged off, cleared cache, removed and reinstalled the add-on, tried again many times, but no joy. I use Firefox version 67.0.4, MacOS High Sierra 10.13.6. I no longer use Ghostery. Wonder if there's a fix in the horizon...
Are you using more than one Pinterest account? No
Are you browsing in private mode? No
Are you using container tabs? No
Not sure what we'd talk about away from here. I'll tell what I do and have been doing for months till this button stopped working: I'd go to my Instagram page, click on a photo/post, click on the Pinterest Save button, a window would open up to give me options to save to my Pinterest account, and that was it—until I started getting the infamous "Sorry, something's not quite right here. Please check that you are logged into Pinterest and try again." I didn't do anything different, it just stopped working.
This doesn't happen with the new Pinterest button I'm using now. I'm guessing it's related to the way Firefox handles add-ons now, and for some reason yours isn't cutting it. Maybe check with them and the changes to their new version?