Some issues. Using Firefox 69.0b3 64-bit (not running Librefox):
-Sometimes on viewing an HTTP page, my Firefox dark theme is partially turned off, turning my UI into a weird mix of black elements and white elements. This doesn't always occur, and re-selecting a different theme then the Firefox dark theme again fixes it until the next time (a browser restart may as well). This seems related or similar to https://github.com/intika/Librefox/issues/126 . Disabling or uninstalling the extension while an HTTP page is open can also trigger this temporary-theme-screwup.
-When multiple Firefox windows are open, if one of them displays an HTTP page then the address bar is turned red in all of them, even though the rest of the windows are displaying HTTPS pages. Also, this addon doesn't update on switching focus between windows, only on switching the current tab within a window, so if you switch focus from a window with an HTTP page to a different window, in which an HTTPS page is open, all windows' location bars will remain red until you switch to a different tab in a window (and vice versa).
-The search bar receives the red background at the same time as well, not just the location bar.
Good addon otherwise, does what it says. It would be nice if the red background color was customizable (and possibly more colors as well). A fair replacement (or possible addition, I suppose) to this addon is turning on 'security.insecure_connection_icon.enabled' and 'security.insecure_connection_text.enabled' in about:config instead, which adds a warning icon and text to the location bar on HTTP pages.

Nice !