Hello lwaldron, go from Tools > Options > General panel > When Firefox starts drop-down > UNselect Show my windows and tabs from last time, click OK to save, exit firefox and restart it.

see for more info : Restore previous session - Configure when Firefox shows your most recent tabs and windows

thank you

Make sure that Firefox closes properly and that there isn't a hanging Firefox or plugin-container process left in the task manager.

Use "Firefox/File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit") to close Firefox if you are currently doing that by clicking the close X on the title bar.

See also:

• "Hang at exit": http://kb.mozillazine.org/Firefox_hangs
• "Firefox hangs when you quit it": https://support.mozilla.org/kb/Firefox+hangs

There are other things that need your attention.

Please Update to the latest Firefox 17.0.x version.

• Help > About Firefox

• https://support.mozilla.org/kb/update-firefox-latest-version

The Firefox Firefox 12.0 version that you currently run is no longer supported with security updates.
It is important to update Firefox and add-ons to the latest versions to get the latest bug fixes and security updates.