First check Firefox is still the default browser -
Tools/Options/Main/"Check if Firefox is default"
If it already is the default, -
Open Windows Explorer or My computer.
Tools | Folder Options | File Types.
Scroll down to URL:HyperText Transfer Protocol and select it.
Click Edit or Advanced, depending on your Windows version.
Select 'open'. Click Edit.
"Application used to perform this action" should read:
"C:\Program Files\Mozilla Firefox\firefox.exe"
(Check the path to Firefox to make sure that is correct and use the
double quotes.) DDE should be checked.