I've installed the newest version of FF on a clean install macOS Sierra. When I expand the program to the full width of my screen I'm seeing double at the top(see attachment). The last time I had issues with FF it was due to a 3rd vendor app running in the background. I've tried stopping dropbox with no luck.

Thanks in advance for any advice:)

Thanks for letting us know Christopher. We will look into getting that fixed for the next release.

Your Welcome...