Since Firefox 4, there is the additional option of completely turning off menu bar and instead using the shiny orange Firefox button which has most common functions grouped together thereby reducing clutter.
Here is how to quickly enable / disable this Firefox button :
1. To enable it, turn off the Menu bar by selecting View > Toolbars and unchecking Menu bar by left clicking on it. This will bring the orange button instead.
2. To disable this Firefox button and bring back Menu bar , left click button and select Options and then left click on Menu bar to enable it.