This is driving me nuts. Win XP (w/Tweak UI), Office 2K, FF 3, all with the same issue. I haven’t been able to find a pattern as to when things start going squirrely, but I end up not being able to use the taskbar to switch between aps. If I’ve got Firefox open/maximized, then click on an office button on the taskbar, it flashes three times and that’s it. I can alt-tab to it without a problem, and if I minimize whatever window is visible I can eventually get to it, but programs just stop switching. Kind of like some other app has an “always on top” option checked. It’s driving me [Crazy Eddie]INSANE[/CE].
Any thoughts?
Happens to me occasionally. I just kill explorer.exe and restart it, in Task Manager. I should say that the hidden but recommended way of cleanly shutting down explorer.exe is to click Start -> Shut Down, then Ctrl+Alt+Shift and click on Cancel. Then Ctrl+Alt+Del, Task Manager, File, New Task, explorer.
Shoot … no luck.
I tried both methods, the “clean” way first. Turned the taskbar b&w for a moment, then it was blank (no app buttons). I went to task manager, selected New Task, and ran “explorer,” and the app buttons reappeared, but I still had the focus problem.
Then I went to Task Manager and killed explorer.exe via the processes tab. Taskbar vanished entirely, I started it up again via new task, it came back with apps, but still no focus switching. 
(missed edit)
Click one sets a button blinking
Click two and I can see the minimization animation (the window getting smaller and collapsing into the button) of the non-focused app
Click three and I can see the maximization animation of the non-focused app
Sometimes – but not always – the third click actually brings an app to focus
RRREEeerrreaarRRR!