Having a close button on each tab in Firefox makes closing tabs discoverable, but at the expense of accidental clicks.
When the tabs get too small the close buttons on inactive tabs are removed, again as a trade-off between discoverability and accidental clicking, but there's still the off-chance.
So for those of us who're comfortable with closing tabs by middle-clicking them, Remove Close Buttons from Tabs removes the close button from every tab, all the time.
Hence the name.
You can get it from Mozilla Add-ons or, if you really trust me that much, directly from me.
It's licensed under the ISC licence:
Copyright © 2009, Greg K Nicholson
Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies.
The software is provided "as is" and the author disclaims all warranties with regard to this software including all implied warranties of merchantability and fitness. In no event shall the author be liable for any special, direct, indirect, or consequential damages or any damages whatsoever resulting from loss of use, data or profits, whether in an action of contract, negligence or other tortious action, arising out of or in connection with the use or performance of this software.
Mmm… shiny simplicity.