I'm old enough to remember when web browsers were a complete novelty. In fact, back in the early 1990s, I ported NCSA Mosaic to Intergraph's long-forgotten CLIX operating system just so I could see what all the fuss was about. Browser technology has advanced hugely since then; modern browsers provide functionality and conveniences that were scarcely dreamed of back then—along with a passel of security problems.
Microsoft Internet Explorer has essentially become the de facto standard for browsers, and it's been the primary browser supported by Microsoft Exchange Server's Outlook Web Access component ever since OWA first appeared in Exchange Server 5.5. OWA has always worked best with IE. Beginning with Exchange 2000 Server, OWA provided two distinct browser experiences. The premium, full-featured client version worked only with IE, and the light, limited-feature version worked with other browsers in addition to IE.
Of course, "works with" and "is supported on" aren't always exactly the same thing. With Exchange 2000, the premium experience required an ActiveX control, which obviously limited its usefulness on non-IE browsers. Exchange 2003 dropped the ActiveX control requirement, but the premium mode was still only supported on IE; Exchange 2007 is the same way.
This limitation hasn't been warmly received by many people. The premium OWA experience is much more pleasant and functional than the light client. Most companies use IE because it comes with Windows, not because they particularly like it. Firefox and Opera have their adherents, and there's still an active market in browser development, as witnessed by Apple's release of Safari for Windows and Google's release of Chrome.
This situation has caused a range of problems for the Exchange Server team. First off, customers have repeatedly, and loudly, asked for support for more browsers. Second, though, they've been asking for support for different browsers than the ones Microsoft might prefer to support.
The amount of work required to add full OWA support for a new browser is roughly equivalent across browsers, so adding full support for, say, three new browsers effectively triples the amount of work required to develop and test OWA—not to mention the requirement to test OWA versions localized for specific languages, too! You also have to factor in the impact of the OS; for example, Safari for Windows and Safari for Mac OS X have different bugs, so just because something works properly on one doesn't mean it will work without modification on the other.
Despite these obstacles, in Exchange 14 Microsoft has committed to supporting the full premium OWA experience in three browsers: Firefox and Safari are being added to the list with IE. As far as I know, the Exchange team hasn't specified which versions or platforms this support will apply to. I'd venture an educated guess that IE 7.x and IE 8.x will be supported, as will Safari 3.x and 4.x for the Mac. It seems sensible to expect support for Firefox 3.x on both Windows and Mac OS X (and maybe other platforms), but without a full announcement, we'll have to wait and see. In the meantime, the Exchange team released a video of OWA support in these three browsers to whet your appetite—take a look at Introducing Outlook Live on TechNet Edge.