Normally, you just have to go to this dialog:

Firefox > Preferences > Tabs

If you have any tab-related add-ons, perhaps they are not fully compatible with Firefox 12? You can review your add-ons and update/disable/remove them here:

Tools > Add-ons > Extensions

Do either of those resolve the issue?

No doesn't help. There is nothing inpreferences and I've disabled all add ons and plugins etc and it's still doing the same??

Any ideas?

In tab preferences, usually you want the first box checked to divert new windows to a tab instead.

To look at the various preferences more directly, you can use the about:config preference editor.

The following assumes that the problem occurs when clicking links.

(1) Type or paste about:config into the urlbar and press Enter. Click the button promising to be careful.

(2) Type or paste browser.link in the filter box, and pause while Firefox filters the list. you should see the following settings relevant to opening links in tabs vs. windows.

browser.link.open_newwindow

3 = divert new window to a new tab (default) (checked*)
2 = allow link to open a new window (unchecked*)
1 = force new window into same tab

* First checkbox in Options > Tabs

browser.link.open_newwindow.restriction

0 = open_newwindow preference honored for all new windows
2 = open_newwindow preference honored for all new windows except script windows with features (default)
1 = open_newwindow preference ignored completely (same effect as open_newwindow=2)

Can you reset these preferences to their defaults?

Try to remove the Firefox plist file (org.mozilla.firefox.plist).

• http://www.macfixit.com/article.php?story=20060606082246983 How .plist files become corrupt and troubleshooting the results

Go to "~/Library/Preferences" and remove the plist file for Firefox (org.mozilla.firefox.plist)