You can drag the New Tab button from the Tab bar on another toolbar and use this code in userChrome.css to make both New Tab buttons visible. Add code to userChrome.css below the @namespace line.

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

/* show New Tab button on toolbar */
#new-tab-button { visibility: visible !important; }

/* show New Tab button on tab bar */
.tabs-newtab-button { visibility: visible !important; }

See also:

• http://kb.mozillazine.org/Editing_configuration

I'm having this same problem, and I'm also having problems getting cor-el's response to work. I originally was able to get the "new tab" button to drag to a new toolbar, but now it's disappeared again (and when I try that nifty trick again, there's no "new tab" button ANYwhere on the screen for me to drag around :-P). And I can't quite figure out where/how I'm supposed to add the code...? Help?

If you do not see any new tab buttons in the customize window then you may have a problem with the file localstore.rdf in the Firefox Profile Folder
You can try to click the "Restore Default Set" button in the View > Toolbars > Customize window to see if that helps.

See also:

• http://kb.mozillazine.org/Corrupt_localstore.rdf

'Found It ...and moved it where I like it! Right click an empty space on your menu bar... > 'customize' > then (I hope) on the far right of my tabs a small '+' sign shows up... drag it to where you want it and drop (in my case to the left of the address bar), and leave it. It looks like a grey camera shape with a '+' sign in the middle.

I tried lepiane's fix, but it moves it to the far right of the browser no matter which bar it is on. Not much different than having it in the same place on the address bar. It's a big mouse move if there aren't many tabs opened. I liked it right next to last tab. Tabs are why I started loving Firefox and not Safari. Then Safari added tabs, but not the new tab button next to the last tab, so I still preferred Firefox. Now it's the same, same. Guess I'll have to get used to using my keyboard to open a new tab, unless there is a fix.

You should be able to move that New Tab button to any position on a toolbar if it is not the tab bar.

If that doesn't work then try:

• http://kb.mozillazine.org/Corrupt_localstore.rdf