UPDATE:
Finally got around to upgrading! The upgrade went flawlessly, and everything seems to be working just fine. It automatically upgraded all of the add-ons/plug-ins I had installed (3 I think), and it told me the version of 1 of the add-ons (PicLens) wasn't compatible with Firefox 3, and asked if I wanted it to check for a compatible version.....I clicked "OK" and it looked, found, and installed the new version of PicLens. It also kept my "saved" log-in info & passwords for various websites intact, so I don't have to type all that cr*p in again...I can just go to the whatever website and click the 'sign-in' button....YEA!!!
It also left my "Toolbars" exactly like I had them set up and laid out, which made me a very happy camper! Especially since everytime I used to upgrade IE it would rearrange the toolbars, delete things I had up there, and just generally p*ss in my Wheaties......Oooh, how I DON'T miss [-]that piece of cr*p[/-] IE!
So if you've been putting off upgrading from FF 2.?? to FF 3 like I had been, don't fear! It's a smooth & simple act that only takes a few minutes to do!