Firefox extension scroll fix

closes #317 (closed)

I was actually unable to reproduce the specific issue described in #317 (closed), as on my system no content is hidden when initially opening the extension popup. However, the scrolling and sizing behavior was also problematic for me locally, and I suspect the cause was the same. I have fixed this issue, and at the same time prevented duplicate scroll bars at default size. It is still possible for duplicate scroll bars to appear at various zoom levels. I believe preventing them entirely would require a more time-intensive fix.