You will probably have an incorrect action set for a content type, it will be incorrectly set to "Use Firefox". For details on fixing it see the "Change the action for a content type" section of the following link - https://support.mozilla.com/kb/Firefox+keeps+opening+many+tabs+or+windows

Thanks. I followed the instructions but could find no file type set to "Use Firefox." I'm concerned that if I reset the action for all file types as instructed I won't know how to reestablish the customized file type actions.