Just an update for anyone else who runs into this problem.

Dann's directions were spot on. I used TortoiseSVN as my visual interface to SVN.

To update the mime types using TortoiseSVN, select all your html files on your local machine, then follow these steps:

(1) Right click and select TortoiseSVN -> Properties

(2) In the properties window, click [Add]

(3) From the drop down box select svn:mimetype

(4) Type text/html as the property value

(5) Click OK - TortoiseSVN will then add the mimetype property to all your local files.

(6) Commit all the altered files to SVN and you're all done!

Your html pages should now work in both IE and Firefox :o)

Cheers,

Anthony