Inspired by this post it occurs to me that I often suffer from the opposite problem. That is, I've got code in a legacy application designed only for Internet Explorer and I need to get it to work in Firefox.

For example, I recently worked on an app that made heavy use of manually simulating click events, like this:

select.options[0].click();

...which completely broke the application in Firefox. But you wouldn't find that information in the answers to the other question, because that's not something you'd ever even attempt if your app first targeted Firefox.

What other things should a developer updating a legacy IE-only application look for when migrating to modern browsers?