We're looking for a unique string--"New Tab"==, so we'll use [http://lxr.mozilla.org LXR's] '''Text Search''' feature. Here are the results you get when you search for "New Tab":

Lots of results, many of which point to comments in the code. However, the first result looks interesting:

Here we see the DTD file describing the key/value pairs for the en-US localized strings. Mozilla uses this technique to allow localizers to translate strings in an application into many different languages without having to change hard-coded strings in the code (you can read more about localization, DTDs, and Entities [http://developer.mozilla.org/en/docs/XUL_Tutorial:Localization here])

Repeating the search with the '''newTab.label''' ENTITY value instead of the "New Tab" string makes a big difference--we have many fewer hits:

Not surprisingly, the first result is the same DTD file (i.e., tabbrowser.dtd) we already found. The second result looks interesting, though:

Here we see the code to generate the pop-up context menu for a tab (i.e., what you get when you right-click on a tab in the browser):

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Armed with this new information, we are even closer to finding the right spot to begin working. We've gone from UI string to XML ENTITY to function. All we have to do now is find that function:

This returns many results for things we aren't interested in, including files rooted in /suite, /db, etc. Since we are interested in finding this behaviour in Firefox, we need to focus on the files rooted in '''/browser'''. One looks particularly interesting:

In this case, the tabbrowser widget has the onnewtab property set to another function, '''BrowserOpenTab();''' (i.e., Firefox seems to handle tab creation in a non-standard way, providing its own method instead of using the default). Since we want to find the definition of this function, we search for '''"function BrowserOpenTab("''', which returns two results:

Again, we're interested in Firefox (i.e., browser) instead of SeaMonkey (i.e., suite), so we skip to the second result:

This shows us that we need to be looking for yet another function, '''loadOneTab()'''. Another search:

The first result is not surprising, and we're back to the tabbrowser widget. The '''loadOneTab''' method calls another method to actually create and insert the new tab:

Since '''addTab''' is a method of '''this''' we can search within the current document (CTRL+F) to find the '''addTab''' method. Finally we've found the right spot!

this.mTabContainer.appendChild(t);