Windows error opening Internet shortcut or local HTML file - Firefox
From MozillaZine Knowledge Base
On some Windows systems, when Firefox is the default browser and is not already running, when you launch it by opening an Internet shortcut, a local HTML file, or a link in another application, two instances of Firefox may open or Firefox opens along with an error dialog, Windows cannot find "<URL or path to file>". Make sure you typed the name correctly, and then try again. To search for a file, click the Start button, and then click Search.. For example, when Firefox is launched by double-clicking an Internet shortcut icon (or by entering the URL via "Start -> Run") you may see an error dialog similar to:
Note: When opening a URL link in a Microsoft Outlook e-mail when Firefox is not running, Firefox may open along with an error dialog titled "Microsoft Office Outlook" with the message, General failure. The URL was: "<URL>". The system cannot find the file specified.  In older Outlook versions (e.g., Outlook 2003) a dialog titled "Locate Link Browser" may appear.  Also, if Firefox is not open and you click a URL link in a document or spreadsheet, Firefox may open a window with a message that the page could not be found with the address bar showing "FirefoxHTML%5CShell%5COpen%5CCommand" appended to the URL. 
Problematic extensions can cause the issue, including the Google Toolbar for Firefox and possibly others (disabling or uninstalling the Google Toolbar extension will stop the error if not caused by some other toolbar).   To see if an extension is causing the issue, try updating all extensions ("Tools -> Add-ons -> Extensions -> Find Updates") in case the problem has been fixed in the latest version. If the problem continues, you can disable or uninstall individual extensions to see if one of them is responsible. Read this if you need more help.
If the problem is still not resolved, try the following solutions (you may need to repeat these steps after each Firefox update):
File Types edit
In Windows XP and Windows 2000:
Note that the "File Types" user interface that allows you to uncheck the "Use DDE" option, as described above, is not available in Windows Vista  so you will need to edit the registry to remove the "ddeexec" key, as shown below.
You will need to manually edit the registry in Windows Vista and above, or in cases where the File Types listing is missing certain entries such as "URL:HyperText Transfer Protocol" (HTTP) and "URL:HyperText Transfer Protocol with Privacy" (HTTPS).
Caution: Editing the registry incorrectly can damage your system. Do not attempt these steps if you are inexperienced or uncomfortable using the Registry Editor.
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