I can't get rid of this new horizontal tab...

I downloaded the new version of Firefox, and now a new horizontal tab has opened right next to my normal tables. It’s a LinkedIn Jobs Insider tab. It has an X to the right that I can click and easily close, but when I open Craigslist, or other job hunting searches, it opens back up again. I’ve tried to click on Toolbars, customize, to try and get rid of. I’ve tried to click on options, and get rid of there also. Can someone please try to help me?
Thank You

Hi,
I could not understand you entirely. Please give me some ScreenShot, upload your image to Imageshack.us or tinypic.com and give me a link here.

Regards