When you install the Turn Off the Lights Firefox extension for the first time, you may see this yellow bar with the following text “a script in this extension is causing Firefox to slow down”.
Here the solution(s) on how to fix this issue in your Mozilla Firefox web browser.
If you can open well the Turn Off the Lights Options page, but you receive this yellow bar when enabling the “AutoStop” feature. Try to disable this option, because there is a bug in Mozilla Firefox performance of the Observer API.
The observer API does not work well on that Firefox web browser. However, there is no issue if you use the Google Chrome, Opera, Safari, and Microsoft Edge web browser.
Check you are using the Firefox version 65 or higher. Because this will show not this yellow bar anymore from Firefox.