Oh, and don't forget that a lot of proprietary corporate apps are written to work ONLY in IE6. I wonder if a "legacy" mode for new versions of IE would alleviate part of this problem. (And yeah, I know that there's a big fat bag of poisonous acid-spitting kitten-eating snakes that this would open up.)
The new auto-upgrade model Mozilla has adopted for FF 4.0 (and that Chrome has had for ages) makes upgrade cycles, for all practical purposes, negligible.
Chrome 6.0 was released on September 2. Since then, Chrome 5.0's market share has dropped to nearly 0%, with Chrome 6.0 picking up all of 5.0's share while also absorbing a good amount of IE's share.
If Microsoft instituted an auto-upgrade path for IE and released updates with more regularity, this would slowly become a thing of the past.
- You could do it as Chrome does and have a per-user install. I don't think Mozilla wants to do this.
- You could install a service or a scheduled task to do the work for you.
- MSI updates can be installed without admin rights if they're Authenticode signed, and Mozilla does have a valid Authenticode digital certificate.