I started using Firefox 3 with the beta 5 release, and I am now using release candidate 1. I have moved several members of my family to Firefox 3 as the opportunity has presented itself. I have generally been happy with it so far.
I really like the zoom feature. I use fairly large screens up to 1600×1200 pixels and many websites are still designed for an 800 pixel wide display. On a large screen this results in a narrow column of a web page, frequently with fonts that are too small to read. The new zoom feature zooms the entire page instead of just the text. The display gets wider and the images also zoom. Everything stays in proportion it is just bigger. This the must have feature of Firefox 3 for me.
Better performance was touted as a important new feature. I had problems with Firefox 2 leaking memory in the early releases. I would have to stop it and restart it to get things moving. In later releases I did not see that problem as much. Firefox 3 has been at least as good as the later Firefox 2 releases for me, but any speed increases have not been obvious to me.
I had problems with installing flash on the beta 5 version, but that seems to have been solved with the release candidate.
Firefox 3 has a new bookmark manager, but I have not yet explored its capabilities. I use at least 4 different computers: my work PC running windows, my MacBook Pro, a windows PC at home, and a Linux PC at home. I want to have my bookmarks synchronized across all of these computers. In the past of I have used Foxmarks and Google bookmarks.
There were problems supporting some of my extensions like the Google toolbar. They have not yet been updated for Firefox 3. Since I rely on the Google toolbar for access to my Google bookmarks, I did not have a real problem with Firefox 3. On the first computer that I used Firefox 3, I simple imported my Google Bookmarks into Firefox 3’s bookmarks. That was fine until I added Firefox 3 to a second computer. There was a version of the del.icio.us Firefox extension that was Firefox 3 ready, so I started using that and have been managing my bookmarks with del.icio.us.
Update: I am really starting to appreciate the location bar. It is very effective way to go to websites that you use a lot. As the number of bookmakrs gets large, picking out the ones that you use a lot can get painful. They are buried in with a pile of others that you might visit only once a month/year. With the location bar I just start typing a drop down list appears with the most frequently visited sites that contain