I am on zen which you can consider to be as vertical tab mode in FF as well (considering zen is based on FF) (but all be it, I love how slick zen looks! Zen is amazing)

And I have the same texture too! I hadn't observed it until your message

Unless I misunderstand something, that texture is not especially related to Firefox or vertical tabs.

I have it both under Firefox or Chromium, and whether my tabs are vertical or not. It's just the website's background.