Comment 73 for bug 417757 I had already done this manually in menu.lst, I now also tried the above - but even though I let update-grub even generate a new menu.lst, the ipv6 param is missing!!!!
I had to add it manually again.

Also, disabling ipv6 this way does solve my firefox DNS problem (I reenabled ipv6 in the firefox prefs), BUT my Ruby on Rails (2.3.4) plus apache and passenger development environment now takes MUCH longer to respond than before. Now my normally much slower laptop (with an identical dev. environment) suddenly is much faster than my PC. Some network lib is still causing a pause before each request - and it is not the apache/passenger side.

It is my mysql connection!! How do I know: On my laptop the vmware-internal network host-only network did not work any longer after the 9.10 upgrade. So I moved the mysql server from the laptop's Windows into the Ubuntu inside Vmware and gave it some more RAM. It now works as before. On my PC - also running Windows and Ubuntu inside VMware - I left the database on Windows and Rails connects over the network, there I don't use host-only networking (the "global" network adapter is always on, unlike on the laptop where I often work without offline any network attached to the laptop).