From
http://www.duncanriley.com/2006/10/08/2-things-i-hate-about-firefox-20/
To hide the “list all tabs” button, you will need to edit the userChrome.css file.
It is located (in WinXP)
C:\Documents and Settings\(user)\Application Data\Mozilla\Firefox\Profiles\XXXXXX.default\chrome\
if you have a userChrome-example.css just rename it to userChrome.css
Add this line to the bottom of the file:
.tabs-alltabs-stack {display: none !important;}
Additionally if you want the the “list all tabs” button shown only when the tabs overflow, use this instead:
.tabbrowser-tabs:not([overflow=”true”]) > .tabs-alltabs-stack {display: none !important;}