Before updating Firefox, try closing or disabling such programs; for example, Spybot Tea Timer and Logitech Quick Cam software have been known to interfere with Firefox updates.
I had tried publishing to a document library, as a content type, trying to modify content types, etc.
You must also have the permissions to create a library or site content type on that site.
Support for libraries and site content types varies, depending on whether your server is running Microsoft Windows Share Point Services 2.0 or Windows Share Point Services 3.0.
It's possible that another running program or a previous instance of Firefox that did not close properly is interfering with the Firefox update process.
This error can also occur if you share your computer with other users, and another user's account is still logged in with Firefox running.