The Catfish platform is designed to work out the box using HTML5 for the editor screen.
While this is great for Android, iOS, Chrome, Firefox, Safari and IE9+ users it doesn't work so well for users with older browsers as you mention below.
For older (non HTML5 compatible) browsers, a window should pop up on site load (homepage) suggesting the user should install Google Chrome Frame.
Chrome Frame is a small web based install which allows the HTML5 content to run in a google chrome area assigned to the editor only.
If some of your customers have a problem with installing this, you can set their storefront to use standard HTML mode, which will run correctly on all broswers, but will be slower to update, and not as mobile aware for handheld devices.
To make this storefront change, login to the affected storefront as admin, and choose:
Admin -> Catfish -> Other -> Editor Settings
There is an option marked "editorsettings.UseDivVersionInsteadFlash:"
Set this to true, and the editor will work in all browsers. Save changes.