Which Firefox is this, what version? The latest is 21. What are the tabs it keeps opening? Are they previously closed ones? More information needed. Don't know if it applies to your situation, but check out this Mozilla article. And I would ignore the malware section, as that's probably designed for Windows users of Firefox.
https://support.mozilla.org/en-US/kb/firefox-repeatedly-opens-empty-tabs-or-wind ows
Also see the recommendation to reset Firefox here. But warning, go to your home folder library>Application Support and make a backup of the Firefox folder before doing this. Option-click and drag that folder to the Desktop. That will make a backup copy.
https://support.mozilla.org/en-US/questions/949086
There are some other suggestions in that new link. Post back if you don't understand how to proceed.
Message was edited by: WZZZ