Thank you for your feedback,

This issue may be caused by the http and https colour settings both being set to green.
You can solve this issue by following these steps:

1) Open the add-ons menu by clicking the striped menu button in the top right corner of the Firefox screen.

2) Select Add-ons.

3) Find the Https highlighter add-on and click the “Options” or “Preferences” button (the wording will vary depending on your Operating System).

4) At the bottom of the Https highlighter page there will be a reset button. This will reset the add-on back to the original settings (green for https, red for http). Click the Reset button.

The default settings will be applied to your next Google Search.
If you are still encountering this issue, feel free to contact me through the support email address httpshighlighter@protonmail.com

Hope that helps.