Considering that Mozilla has been ignoring the desire of a lot of users to remove the extensions button or to give us the option to remove it, I think that they should at the very least redesign it. While the icon itself is indeed centered in comparison to the other extension buttons, the icon itself looks off-centered in comparison to the hamburger button of the application menu, and once you notice it it's very difficult to ignore.
The icon should be redesigned in a way that would make it look more in place, perhaps by making it smaller like Chrome does it, or the Application Menu icon should be redesigned to better fit it.