Really excellent core functionality--saves me a lot of time on videos! However, its Chrome-first heritage is evident. A couple of tweaks to function better in Firefox would be helpful:

1 - logging in to your account is not possible from within the open Scribe sidebar; the popup does not appear so you have to log in through the Scribe homepage

2 - Scribe halfway breaks the Firefox sidebar button: Firefox's sidebar houses bookmarks, history, etc and also extensions like Scribe. Clicking the sidebar button normally opens the sidebar to whatever module was last focused there, with the option thereafter to choose a different module from a drop-down menu at the top of the sidebar. But if Scribe was the last item focused, and you press the sidebar button while a non-YouTube website is focused, the sidebar opens only for a fraction of a second and then closes itself again.

Edit to respond to developer's question: An overlaid window sounds like it would perhaps be too much of a compromise--since it would block parts of the YouTube page. Instead, if the sidebar button is pressed and Scribe was the last module used and a non-YouTube site is focused, keep the sidebar open and Scribe could display a message like "go to a YouTube video to see a transcript."