Comment 7 for bug 436190

#I found a fix:
$ sudo aptitude remove xulrunner-1.9.1

# Then I said "n" (no) to the first three proposed solutions from aptitude, (no idea if they would have worked) and eventually was presented with

Downgrade the following packages:
xulrunner-1.9.1 [1.9.1.9+nobinonly-0ubuntu0.9.10.1 (karmic-updates,
karmic-security, now) -> 1.9.1.3+build1+nobinonly-0ubuntu6 (karmic)]

Score is -9772

Accept this solution? [Y/n/q/?] y
The following packages will be DOWNGRADED:
  xulrunner-1.9.1
The following packages will be REMOVED:
  firefox-3.5-gnome-support{a} firefox-gnome-support{a}
  xulrunner-1.9.1-gnome-support{a}
0 packages upgraded, 0 newly installed, 1 downgraded, 3 to remove and 0 not upgraded.
Need to get 7,986kB of archives. After unpacking 524kB will be freed.
Do you want to continue? [Y/n/?] y

and now firefox-3.5 (.9) works again. (Still, this isn't good news for Ubuntu or normal users. I still have no idea which update put my install in this position, or if it was something that I did.)