IBM has published a lengthy article explaining how you can make your IE only site work in e.g. Firefox, Opera or Safari.
The article handles browser, DOM, JavaScript, CSS, Event and XML differences. It even touches Quirks vs. Standards mode and Rich Text Editing.
Ever have trouble getting your Internet Explorer-specific Web applications to work with Mozilla? This article covers common issues associated with migrating applications to the open source Mozilla-based browser. You’ll first learn basic cross-browser development techniques, and then develop strategies for overcoming the differences between Mozilla and Internet Explorer.