Mozilla Labs has released a new test result which was used to find the most popular Firefox menu buttons, according to the initial findings, Close Tab is the most popular button used in Firefox, followed by New Tab, Bookmark Item, Back and Reload.
However, according to further in-depth analysis it was found that most people used keyboard Shortcuts to close tabs and the mouse more often to bookmark items and copy paste content. You will also find a screenshot of menu usage, grouped by the menus in Firefox, in the screenshot below.
An interesting thing that I could see in the data presented is that the New Tab operation is pretty less as compared to the close tab action, which suggests that users are either clicking on links on website, or using their bookmarks to open almost 50% of the tabs they are browsing.
More details on the most popular Firefox menu buttons and their usage can be found at Mozilla Labs.