Re: how to install firefox v6 without losing bookmarks and addons from v3?
If it is done as an update, you would be using the same profile.
When reinstalling Firefox from a download (http://www. mozilla.com), Firefox must be down once the installation starts. When the installation finishes, don't let the install start firefox for you. Instead end the install and start Firefox in your normal manner, thus preventing creation of a new profile which does not have your bookmarks cookies, etc (but your old profile would still be around and would).
After the install completes start Firefox in you normal manner.
Get in the habit of closing Firefox properly, by using File > Exit, or the new Firefox button > Exit, and not by closing with the "X" in the upper right which just closes the Window. When the installation starts and you have properly attempted to close Firefox, you may still have to check that Firefox is down through the Windows Task Manager ("Ctrl+Shift+Esc" on Windows 7) using the Processes tab. Closing Firefox properly will usually work but sometimes it will still hang in there due to sub processes.
Please mark "Solved" one answer that will best help others with a similar problem -- hope this was it.