Thank you for your report. This addon has no effect on fullscreen vs. windowed, and it is also the first report of such an issue. It is possible you might have a conflict with another addon, or even a video driver problem. Does the issue persist with a clean install of the latest Firefox and/or disabling any other addons? After your report, I've tested this addon on both Windows and Linux and have no problems with fullscreen. If trying these steps resolves your issue, or further assistance is needed, please consider a follow-up and an update to your rating. It would be greatly appreciated.