Upgrading Firefox from 3.0 to 3.5 on Ubuntu proved to be unusually awkward, however I got there in the end.
As 3.5 is now in the Ubuntu repositories I simply selected it in Synaptic Package Manager, clicked "apply" and it installed smoothly, however when I ran Firefox from the icon it actually launched Firefox 3.0 - not the shiny new 3.5 I was expecting. This was fixed by changing the symbolic link to point to the right version.
In the /usr/bin directory I ran "sudo rm firefox" to remove the existing link and "sudo ln firefox-3.5 firefox" to create the new link, allowing Firefox 3.5 to load as default.
So whilst happily launching Firefox 3.5 I now got an error message stating "XML Parsing Error: undefined entity Location: chrome://browser/content/browser.xul Line Number 34, Column 1" and of course Firefox crashed out.
To fix this I deleted the existing profile by running "mv .mozilla .mozilla.old" in the home directory "/home/yourname".
So after two quick fixes it's up and running and appears to be quite good.
I decided to blog this as otherwise I'd end up repeating the procedure and wonder how I fixed it last time :)