I had a closer look at the errors now and I guess I'll have to be absolutely honest now about what I did to upgrade to Squeeze since easy standard steps don't seem to work.

Because the watchdog always rebooted my N900 during dist-upgrade attempts I first upgraded small chunks of packages (30-50MB download size) via debbie aptitude.
When I realized that nice could keep the watchdog calm and switched to that procedure half of my Easy Debian was already upgraded (yes I had lots of time that day ). The whole X-server related stuff and probably even Easy Debian's dbus was a part of these chunks.

No idea if that will solve the problem but I think it's worth a try to manually upgrade dbus first. I can't even exclude that I upgraded pulseaudio from within the Easy Debian desktop (which might affect the dist-upgrade since Easy Debian's dbus is actually running) since I did a lot of tinkering with pulseaudio back then.

If that still doesn't work I'll try to reproduce the dist-upgrade with a stock Easy Debian installation while keeping the procedure as simply as possible and documenting the steps exactly.
But most likely I won't find the time for that before x-mas.

As for your firefox warning, I get that too. Eventually I just uninstalled the package that caused the popup (the commandline output still remains). Since it seems to work except for that warning I didn't investigate it any further. I'll add that to the regression list in the wiki article once I've identified the package again.