Why not add wiping Firefox's cache to Browser Wipe's capability?

For one simple reason: Firefox can be set to never cache any files onto the hard drive in the first place, so if properly configured for security, there are no cached files that need wiping to maintain privacy. Microsoft Internet Explorer cannot be set to never cache any files onto the hard drive. Microsoft Internet Explorer requires that at least 8MB of hard drive space be used for caching files.

You can set Firefox to never cache any files by doing the following:

1. To set Firefox to never cache any files onto the hard drive, use the Tools menu.
2. Select the Advanced Setting.
3. Select the Network Tab.
4. Set the "Offline Storage" amount to 0MB as shown below.
   
5. Hit "Clear Now". This will delete (but not wipe) any files Firefox presently has stored in its cache on your hard drive.
6. Also make sure the "Tell me when a website asks to store data for offline use." box is checked.
7. Hit OK.

I'd also suggest that Firefox be set to delete all cookies on exit (under the privacy tab) and that extensions such as Ghostery, HTTPS-Everywhere and NoScript add-ons be installed and properly configured.