(Edit: My post body seemed to have been replaced by the title?)
The browser chrome uses transparency so that it's colorized by the content behind the browser. This looks really great when the browser shares a desktop with other apps, but becomes distracting when in full-screen mode with nothing to block a colorful background from making the tab sidebar too colorful. It'd be great to be able to disable transparency in full-screen mode. iTerm2 also has this feature, which I appreciate.