That seems a little extreme... I don't even have an exception for pinterest.com and I'm not getting a block.

Are you using the Pinterest extension (red button on main toolbar) or the Pinterest bookmarklet (appears on the Bookmarks Toolbar)?

Do you have any add-on popup blockers that might be applying a stricter approach than Firefox's built-in popup blocker?

the Pinterest extension from here (The Pin It button): https://about.pinterest.com/en/goodies.

also this one: https://addons.mozilla.org/en-US/firefox/addon/pinterest-pin-button

The problem happens with both of them until i add that specific site to the exceptions list. The first one i couldn't find on the add-ons site, only on Pinterest, but it does install an add-on named 'Pin It Button 1.3.6' by Pinterest, Inc. I get the warning at the top of the browser window "Firefox prevented this site from opening a pop-up window."

I definitely didn't install anything specifically pop-up related and did not install any add-ons at all recently. This just started happening after upgrading to Firefox 38. However, it is possible that another add-on updated automatically and that caused the issue (I do use ABP, NoScript, & FlashStopper). i could try this with all add-ons disabled except for the Pin It Button and see if it still happens.

I had 1.35 from goodies and updated to 1.36 (https://about.pinterest.com/en/browser-button). Seems to work the same. I didn't try the other one you mentioned. I use NoScript, so I don't think that one necessarily interferes, unless there's a setting I turned off.

After more pinning & testing I noticed that the problem doesn't happen with every site I pin an image from, only certain sites, but I haven't found any noticeable pattern, and the problem still occurs with most sites.

Also, version 1.37.9 of the Pin It Button is available now at that 'Goodies' link. Before it wasn't auto-updating for me.