I recently had to re-install my PC and I decided to take the opportunity to freshen up my Firefox profile by abandoning my prefs.js file and only migrating settings and extensions as needed.
As part of this, I ran into an issue I dealt with a while back and had to look up once more.
I don't like having my little red X to close the tabs actually in the tab. I like it at the end of the tab-bar. Just a quick rundown of how to change this:
Browse to about:config and filter for browser.tabs.closeButtons. Set that value to 3.