Rated 5 out of 5 stars

Hi, I just started using this add-on and it's exactly what I was looking for since many years. Absolutely wonderful job!
May I suggest that you add the option for users to choose an alternative icon style? For example, I use a light grey Firefox theme with minimalistic dark grey icons (with transparent icon background) and the green - and + icons of URL flipper are the only ones that disrupt the overall clean look of my Firefox. Any chance that you make the - and + colored in RGB 73, 73, 73 and having transparent button background? That would also work with many people's custom themes since that dark grey color I mentioned is neutral and pretty much looks fine when using any other color except the very dark themes (which is a good idea to have yet another option for light grey - and +), while the transparent button background contributes to the purity of the custom theme. Hopefully this could be done as I really like your extension. Cheers!