This happened once to me a couple of years ago. Try going folder options->file types->advanced, then click "new," type in "open," browse for Firefox.exe, and select it as the program to perform the action. Then "ok" your way out, and check the shortcut again.
If that doesn't work, go back under file types, select each problematic extension, and press ''delete.'' Then click ''new,'' type in the extension, click ''advanced,'' and select the associated file type. Once they are recreated, see if you can then press ''change'' and set it to Firefox again.
Hope this helps,
This shortcut worked fine when using firefox and when I installed and made avant browser default. I decided to go back to firefox as default. Now when clicking the icon the print page opens. I uninstalled avant and removed all references in the registry. The file types in the folders options shows firefox as the program to open html files. The http file type shows none and the change button is grayed out. When trying to enable firefox in the advanced tab it still shows none. When clicking a link in outlook express firefox opens. Any suggestions what else to look for?