First, you need to ensure that the file associations for VCS files are set up correctly. Here are the instructions for doing this on Windows XP. You will likely need to perform both sets of actions.

To change which program starts when you double-click a file whose file type is associated with a particular program, follow these steps:
1. Open My Computer or Windows Explorer, right-click a file for which you want to change the program that opens that file type, and then use either of the following methods to change which program starts:
2. Click Open With to choose the program that you want.
3. Point to Open With, and then click Choose Program to choose the program that you want.
4. Notice that the Open With dialog box appears. Use one of the following methods to select the program that you want to use to open this kind of file:
5. In the Programs list, click the program that you want.
6. Click Browse, locate and then click the program that you want to use, and then click OK.
7. Click Look for the appropriate program on the Web to browse the Internet for the program that you want.
8. Click to select the Always use the selected program to open this kind of file check box if it is not already selected.
9. Click OK.

To change which program opens a file when you download it using Firefox:

1. Click on the Tools menu item and select Options.
2. Click on the Content Icon
3. In the File Types section click on the manage button
4. Find and update the VCS entry, if there is one.