The french web site Le Journal du Net has a story about Firefox 3 that contains "feedback" from users. I am putting "feedback" between quotes because some of the feedback seems to me highly questionable. Anyway, since it's in french, and since it's useful input anyway, you'll find a summary of the article in english below. Warning, it's really a summary, I did not test, I have no opinion myself, I am just quoting the article :

Pros

  • cleaner UI
  • faster engine
  • awesome bar
  • non modal passwd manager
  • spell checker
  • better tab management
  • less crashes, better stability
  • "the best browser, IE is dead"

Cons

  • not a lot of visible changes
  • some useless changes
  • download manager's UI
  • some sites don't work correctly with FF 3
    • Hotmail shows the "classic" interface and not the new one
  • too many extensions not ready for FF3
  • some sites are detected as "malware sites" even if they're not
    • Support to Ingrid Betancourt's web site
  • some sites give a 500 error with FF3 and work with FF2
    • http://www.assedic.fr/ (the authenticated part of the site, I did not test it)
  • FF3 chokes on some images in a web page while FF2 works correctly
  • often crashes with Flash
  • copy/paste often broken (uuuh ?!!?)
  • page scrolling sometimes not smooth

Back to my own personal experience with Firefox 3 on WinXP,there are only two things that annoy me (only a very little bit) from time to time, but I could not investigate yet so I did not file a bug :

  1. from time to time, the caret is visible in the page and that harms scrolling using arrow keys ; I have no idea if this is related to an extension or not, but I don't think so since I have no extension playing with designMode or contentEditable and I have only the following extensions running: AdBlock+, DOMI, Glazoom, Greasemonkey, HTML Validator, Venkman, SQLite Manager, Webchunks.
  2. from time to time, in particular when I close a window containing multiple tabs and one of these tabs has flash content or a video running , Firefox 3 remains in memory and eats up to 90% cpu.