Everything will still be there once you upgraded to FF 4. The only problem is that some add-ons/plug-ins might not be compatible. There's nothing Mozilla can do, the developers of these add-ons/plug-ins will have to update them accordingly.

Extensions may need updates to work with Firefox. But your passwords, bookmarks, form data will be preserved between the versions.

I just did this also on one of my computers and all compatible add-ons will transfer automatically. You may have to go in and change some toolbars and things since Firefox 4.0 is laid out differently than 3.6.15 was, but it is well worth it. I installed the Firefox 4.0 Beta about a month ago after a Windows Service Pack 1 update totally disabled my computer and I had to start from scratch. Firefox 4.0 is laid out more similar to Google Chrome type look making it a lot cleaner and it is way way way faster than the previous Firefox version. Not quite as fast as Google Chrome is but the add-ons are much more prevalent in Firefox than Chrome making it a nice customized internet experience on your computer. Some add-ons that are not compatible will not transfer over but you will be told which ones they are and some of them all you will have to do is update them and then they will work. When I installed the Beta a month ago Fastest Fox and Tabs Mix Plus were not available but they are now. So most of your add-ons will soon be available. I highly recommend you make the change. It's well worth it for speed and a much cleaner look.