See my previous posts on this topic:
Now, with Firefox version 24.0 we have yet another issue. With the Google Toolbar Add-On extension installed and enabled, you can no longer open a new tab in Firefox. You can open a second instance of Firefox, but when you click on the New Tab button (the + sign) or press Ctl-T nothing happens. Oddly, if you disable the Google Toolbar for Firefox extension, then suddenly all of your new tabs you previously requested suddenly appear.
The only work-around found so far is to press the CTRL key as you click a link or right-click a link and choose, "Open Link in New Tab". Once the new Tab is open, then use that tab for what you need. Another possibility is to downgrade to Firefox version 23.
Another tip. Type about:blank in the address bar, hit enter, which gives you a blank page, and then save that in the Bookmarks Toolbar, now you have a quicker way to right-click on a link to give you a new Tab to work with.
Update (10/2/2013)
Many thanks to John Tombs who alerted me to a fix (workaround) to this issue.
In a post on this forum: https://support.mozilla.org/en-US/questions/967747#answer-485381 a user called WhopperCock said:
"Type about:config into your browser, then type browser.newtab.preload into the long search bar that appears, then set the Value to False, restart browser and it should be working fine just like it did for me."This worked perfectly for me. Hope it will help others as well. And thank you WhopperCock, whomever you are, for posting the fix.