As Microsoft's Internet Explorer browser is, as of 2002, the de-facto standard web browser, this poses real problems for the developers of more standards-compliant browsers such as Mozilla. If they adopt the Microsoft extensions to the DOM, they risk losing any credibility in their calls for web-sites to become standards-compliant, and if they do not, they risk alienating their users by losing much or all of the content of web-sites which use the non-standard extensions. Cynics regard this as another case of Microsoft applying embrace, extend and extinguish tactics. This could be considered ironic, as both Microsoft and Netscape were guilty of supplying non-standard features in an arms-race for standards control, and Mozilla is a Netscape initiative.
The general consensus appears to be that this will only change if new standards-compliant browsers gain a significant market-share on the Web, thus making the use of non-standard extensions a commercial problem for the authors of non-standards-compliant websites.
Internet Explorer 7.0 is rumoured to be likely to be more standards-compliant in its implementation of the DOM, but is unlikely to pull support for the current proprietary DOM extensions. Time will tell.