Two comments:
1) I think you underestimate the importance of Firefox in Microsoft's decline in importance. It is Firefox that makes sure that all those Web-based apps aren't held hostage to Microsoft's implementation of web standards.
2) Whilst its all about the web it isn't all about the browser. Ajax-y web apps show the potential for ignoring platform specific APIs (such as Win32)--especially when you don't try to mimic platform specific behaviour (the big problem with Swing-based apps: stop pretending to be a native desktop app! Rejoice in being a x-platform web app!). But ajax inside Firefox/Safari isn't going to be enough for me to abandon all of my 'rich' Mac OS X apps: if you want me to put all of my brain in your web-based wiki or PIM, give the same 'richness' and sophistication of design that I can get with VoodooPad, DevonThink, and Curio.