Despite the slew of other reviewers demonizing the Firefox Devs, it is abundantly clear that their issues are user errors not dev errors. :e: Turns out there is an error, but that doesn't rescind the fact that the prior reviewers are still dicks about it :)

I just installed the app, have used it perfectly fine.

I am logged into a single Pinterest account, and am running no other add-ons. As a note, I run very stringent privacy options and restrictions on both my device and Firefox, and I'm still not plagued with any issues.

Thank you Firefox Devs for being patient and doing your best, this is working great for me and my design work!

:e: I don't see a way to respond to the dev response below, so I'm editing it in.
I am running container tabs, but specifically I only had a Facebook container tab open at that time. To my knowledge I didn't have anything else actively running in a container tab, except for the Facebook one.

:e: I now got the login error myself, here's a capture of the error message & the error log text:
https://imgur.com/a/mJ3gBJR - Error capture
"Invalid URL
The requested URL "[no URL]", is invalid.

Reference #9.ae9a3b17.1562655664.4f2c2b8a " - Error log text
I was currently logged into Pinterest on a Tab, and I Reloaded the tab and still had the error. I was attempting to pin an image from Unsplash when I encountered the error.