Here's an update on Firefox support:
When we started (around 2013) it was pretty clear that we did not want to create a Firefox extension based on XUL (which is now obsolete) That was the right decision as we would have wasted important resources early on for technology without a future.
During the following years the situation and future of Firefox was unclear. Although from a technological point of view things are more stable now the situation is still unclear. The product is good, there is lots of marketing hype and hope.
But it does not change the fact that still more Safari users would like to use Paperpile than Firefox users. It also does not change the fact that Paperpile is a complex App which can't simply "re-packaged" for another browser. It's not just that Paperpile uses Extension APIs that are not available on Firefox it's also that we use HTML5 features that differ completely between the browsers.
So we will not be able to port the existing extension to Firefox. Instead we will develop Paperpile in a direction that will make it possible in the future to use Paperpile with other browser. That's our long term goal.
Our short term goal is to make Paperpile available on every operation system: Via Chrome on Mac, Windows, Linux and native on iOS and Android. That's very realistic, while having Paperpile on every operating system AND every browser is not our goal at the moment.