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

More info Windows XP and firefox 3.6.21. I have a windows 7 machine and whenever I click on a bookmark, it opens in a new tab automatically. On this machine, I have to right click on the bookmark and select "open in new tab".

Thanks for the reply. The Ask toolbar is not shown in my list of extensions or plugins and it is not listed in the add/remove programs list.

Well, it is showing as being install in the version of Firefox that you posted here with.

Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.21) Gecko/20110830 AskTbAD3/3.12.2.16749 Firefox/3.6.21

"and whenever I click on a bookmark, it opens in a new tab automatically"

Probably caused by an extension, or a couple of extensions that are in conflict with each other.
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

Oops, my bad. I was looking on the wrong computer! When I looked on the problem computer, I found both. I removed them. Problem solved

Thanks again, Jim