simples e prático.

Haven't tried it yet

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

Love it very much. But it isn't working already for about a month :(

good

Was working great then today had a message pop-up on Firefox saying: some add-ons have been disabled, the following add-ons have not been verified by use in Firefox. Whilst it didn't actually show any add-ons after the message, I did notice that the Pinterest add-on was suddenly missing. And like everyone else, I get a connection error when trying to download it, despite my internet connection working just fine. Clearly someone's done something to introduce a major bug!