Firefox 12.0, which you posted here with, is 5 versions behind the current Firefox 17.0.1 version.

Zotero support is over here - http://www.zotero.org/support/start

The Zotero support forum is here - http://forums.zotero.org/categories/

The Zotero support site will only tell you that it is a Firefox problem. They send you here. I agree that it is a Firefox problem. It is Firefox that is deleting Zotero. I had weeks of research material saved in Zotero for a college paper that is due in two days. This is devastating!

BTW I have the newest version of Firefox and this problem just happened.

Is the Zotero add-on enabled in Firefox?

If not, is the Zotero 4.0.3 version Firefox add-on installed?
The Zotero version available from the Addons Mozilla dot Org (AMO) website isn't the latest - https://addons.mozilla.org/en-US/firefox/addon/zotero/ - so it may not update automatically.

Get it here:
http://www.zotero.org/download/

Thanks. Downloaded it. Now I have my fingers crossed.