some advice Rated 5 out of 5 stars
I hope that there is a option that we can control the blank between two Button.This review is for a previous version of the add-on (1.0.9).
You can try the newest devbuild v1.1pre2 from versions page https://addons.mozilla.org/addon/cstbb/versions
I've added an experimental setting to change the space between toolbar buttons that are placed on navigation toolbar.
Since sliders are not allowed for inline settings I used a menulist with predefined widths (-1px up to 20px).