When Firefox is set as the default browser on a Windows machine, an error such as the following may occur when trying to open the language reference files locally using the F1 shortcut key:

Firefox can't find the file at /C:/Documents.

In order to resolve the issue, go to Preferences->Application->Documentation and select the "Web site" radio button. After doing so, a URL needs to be placed in the field, which points to the the local folder containing the HTML files. For instance, if the documentation is located in:

C:/Program Files/4D/2004/documentation

Place the following in the field:

file:///C:/Program Files/4D/2004/documentation

Click the OK button to apply the changes.