Just upgraded to Karmic Koala, and everything appeared to go fine.. But when I tried to run firefox – I got XML Parse error, browser.xul undefined entity.. Turns out it was getting confused with some xul for Knife Sharpener Reviews from an old install of firefox-3.
Tried a reinstall of firefox-3.5.3 which did no good.
Anyways – the way to fix this particular problem is just to delete the .mozilla folder in your home directory. Obviously – you need to be careful with your bookmarks etc – but really you should be using a bookmark backup service like xmarks anyway!