I also experienced the bug with nothing happening after clicking the button in toolbar (and no button appearing on images at all), turns out it was caused by FF content blocking (I also use container tabs for almost every tab and they cause no issue at all)
Edit:
The solution is to just turn the content blocking off for sites you wish to use Pinterest Save Button on, how to do that is here: https://support.mozilla.org/en-US/kb/content-blocking#w_turn-content-blocking-off-on-individual-sites, or turning the content blocking off completely, like described here: https://support.mozilla.org/en-US/kb/content-blocking#w_how-to-block-more-or-fewer-trackers