I got the same problem with Firefox 3.6.15 after I updated the Zotero add-on to the version 2.0.9. If I turn the Zotero off, there is no prompt on starting the Firofox, when it is turned on again, the prompt comes back. This version of Zotero might be related with the problem.

Well spotted! I have been annoyed by this error. Same version of Firefox and of Zotero. Disabling Zotero stops the error message from appearing, here too, so Zotero is likely the culprit. I suppose we'll have to put up with it until Zotero fixes it.

Thanks a lot guys...it is indeed a problem with Zotero. I also notices that if I install MaComfort it solves the problem. But I don't want a demo version (or full) of something in my computer which I don't want so I unistalled Macomfort after checking. So probably Zotero people are the best to solve the problem. It is not much of an annoyance..you just have to click once extra :). And Zotero is such a wonderful tool that this "annoyance" can be tolerated! Hope someones already pointed it out in the zotero community.

Please have a look at: https://addons.mozilla.org/en-US/firefox/addon/zoteroquicklook/ Details are provided why zoteroquicklook add-on needs maComfort (or Gloobus for Linux systems) and how you custom it if do not want to install or use them. Alternatively, you may disable the ZoteroQuickLook add-on, while the Zotero still will function, you can not use the quick look preview, but it resolves the problem of the prompt in issue on starting the Firefox.