The answer, apparently, is “yes,” at least according to this post by an attendee at New Zealand Foo Camp (aka Baa Camp):
Firefox3 is going to deliver support for offline applications.
Why is this important? Because when you go offline you will still be able to interact with your applications. So in a webmail scenario, read your mail, write drafts. Web Calendars would work.
More importantly imagine the opportunity for Line of Business Applications. The Browser really does become the Operating System – with persistent storage.
This will allows richer SaaS applications and goes some what towards eliminating the offline scenario issue of web based applications. This makes Web apps even more compelling.
Plenty of people have pointed out that this kind of thing is already possible using the Dojo offline toolkit (for example). That just highlights the power of Firefox as the client-side platform of choice for developers.
Plugins and third-party toolkits are fine, but until it’s built into Firefox, it won’t be broadly adopted.
Source: Rod Drury > Firefox3: Web Apps Game changer (via R/WW)