I did a test in chrome and firefox, chrome with no extension, firefox with a dozen.
The test is form goolge (perhaps is optimized for chrome?), here and the result was:
Firefox: 118
Chrome: 2479
So I wonder: the benchmark is also influenced by the installed extensions or chrome is really so much higher than Firefox in performance?
Yes extensions affect performance… Especially in IE7/IE8.
As a general rule, use whatever extensions you want/need but consider that every one of them may affect your overall performance.
Some extensions however (e.g. Firebug) may slow down Firefox… but their assistance in developing web sites / applications far outweighs any performance hit.