At my place of employment, we are increasingly finding it difficult to develop for IE, which was historically the easiest browser to target, from an intranet-app point of view.

1. It was already deployed.
2. It already understood NTLM authentication, thus well integrated with our domain-level security.
3. It had neat, albeit non-standard features such as XMLDOM and XmlHTTP.

Now, we are increasingly irritated by issues presented by IE:

1. There are several versions: IE 7, 8, and soon 9 beta, which all have slightly different issues related to performance, functionality (especially re:security and zones), and aesthetics.
2. IE 7 and 8 are slower than Webkit-based browsers. Period.
3. There are technology limitations such as missing canvas element, CSS bugs, etc. that make it hard to use 3rd party packages or even consistently write code across IE versions.
4. Users are increasingly using Firefox or Chrome, even for intranet use.

Does anyone have experience with making a transition?

Any advice would be welcome.