Words of advice from the multi-platform add-on session at AddOnCon:
- Start from the beginning to support multiple browsers by not using features that are specific to one or the other.
- Keep things like XPath expression standard
- Test IE add-ons on Vista and IE8 first--especially if you're going to use an external process.
- Using an external process in IE can greatly increase testability
- Modularity and code portability
- The learning curve for IE add-on development is steep. Once you're on top, it flattens out and gets easier.
- Crashing the browser is easy in IE and the opportunity for conflicts is high
- Make sure your code can generate debug reports and give the user the opportunity to forward them (debughelp.dll is your friend)