Currently with Firefox 66.0 (64-bit) browser button update confirmation page keeps loading at browser startup. This only happens though if the browsers cache is cleared. Still though while even the browser button add-on is the most up to date version (4.0.75) the confirmation page still loads: https://about.pinterest.com/en/browser-button-confirmation-page?xuid=Ricd7jdYpDt1&xv=ff4.0.75
Nevertheless though it is still a great extension to bring content into Pinterest from other sites. Sadly though there seems to be a growing number of websites that are rich with content that are blocking users from pining content. So when a user sees the Pinterest button greyed out this may very well be the case of the website blocking the save button extension and not necessarily an issue with the save button itself. Have noticed various Flickr accounts whereby their licensing settings on their content does not allow images to be pinned (button greyed out), though interestingly these Flickr accounts were discovered by following content on Pinterest to the source link on Flickr. Hope this dosn't create issues for Pinterest in the future...
Edit: Using multiple different cleaning utilities to clear cache and other locations that are not cleared by Firefox's default operations. Currently using CCleaner Pro and Wise Disk Cleaner. In Ccleaner (v5.55.7108) the following settings are checked in regards to Firefox cleaning: Internet Cache, Internet History, Cookies, Download History, Last Download Location, Session, Site Preferences, Saved Form Information and Compact Databases. Unchecked is Saved Passwords. No other additional settings in regards to Firefox are set.
In Wise Disk Cleaner (v10.1.4.760) the following settings are checked in regards to Firefox cleaning: Under files section, Web Browser Cache - Mozilla Firefox Cache is checked.
Under Traces Section, Web Browser Trace - Mozilla Firefox Other Histories is checked. Between the two programs both handle session store data differently as of now and require both programs to remove session data completely. I might start by creating a new Firefox profile folder and see if this dosen't correct the issue, perhaps there is a mismatch in version data stored in a database file. That may very well be the case since seemingly even though the extension is up to date the confirmation page still loads as if the extension was just updated.