There are many ways of opening a new tab, which one does not work for you? The method I generally use is Ctrl+T See http://dmcritchie.mvps.org/firefox/tabs.htm#newtab

With Firefox 4.0 as distributed there is no space to the right of the tabs to double click to open a new tab. The new tab button is a [+] sign at the far right. see item # 4 "Toolbar buttons moved to other side and combined" in the following.

You can make Firefox 4.0.1 look like Firefox 3.6.17, see numbered items 1-10 in the following topic Fix Firefox 4.0 toolbar user interface, problems (Make Firefox 4.0 look like 3.6)

Have tried the controlT, double right click and hit the plus nothing works.

The Ask Toolbar is causing that in the Firefox 3.6.13+ versions. Uninstall that extension.

There are a couple of places to check for the Ask toolbar:

• Check the Windows Control panel for the Ask Toolbar - http://about.ask.com/apn/toolbar/docs/default/faq/en/ff/index.html#na4
• Also check your list of extensions, you may be able to uninstall it from there - https://support.mozilla.com/kb/Uninstalling+add-ons

I removed the older version of Firefox and downloaded the newer version and all is working. Thanks for all the suggestions.