Firefox will do that with large layouts especially when there are lots of images, it has to first load the page and all of the images into the browser, then it can apply the style sheet to it.
Because of all of the images that need to be downloaded to the browser for this page, it is taking some time for the page to load in the browser. Only when all of the images are downloaded can the page styles be applied to lay the page out.
The layout you are using was created with SiteAssist version 3, not SiteAssist Pro.
If you where to recreate the site with SiteAssist pro, you would need to start from scratch, you will not be able to import your existing layout into SiteAssist Pro.