How to get the smaller tabs back in Safari 6?
The glims tool - available from http://www.machangout.com - should do the trick to re-slice the tab width to something fixed if you prefer the old look.
If you have SIMBL installed, download cosyTabs (Lion & Mountain Lion) and your old-style tabs will be back. (Here'sa direct-download link.)
Open the zip and place the .bundle file in: /Library/Application Support/SIMBL/Plugins/
Works for me and SIMBL never gave me any trouble. On the website it says that it's only for Mountain Lion, but it works on my Lion Mac as well.
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