For those wondering whether to upgrade from Firefox 3 to Firefox 4 I'm happy to say all went well here under Windows 7 x64. My first action before upgrading was, of course, to check that all my current Extensions were v4 compatible. The Norton Internet Security Toolbar was the only incompatible extension of note but as I had that disabled anyway it's no loss to me. The Microsoft HTML 5 extension for v3 is also incompatible but I'm guessing that it isn't needed for v4.
The only gotcha
I suffered was not being able to find quick access to my "Bookmarks" with the Firefox menu bar hidden. Found it eventually at the far right of my Bookmarks Toolbar so all seems to be well except I'd have preferred it on the left! Version 4 does seem a little snappier too.
Fixed the "Bookmarks Button" issue. If one doesn't display the Bookmarks Toolbar (View options) then a Bookmarks Button appears in the Firefox 4 Navigation Toolbar. Makes sense of course but when the Bookmarks Toolbar is displayed the Bookmarks Button, by default, moves from the Navigation Toolbar to the right of the Bookmarks Toolbar. Some will like it there but I don't because with multiple nested folders of bookmarks the pop-up menus start appearing to the left of the parent when screen real estate runs out.
The trick to getting the Bookmarks Button at the left of Bookmarks Toolbar is to note that when you bring up the Customise dialog box the icon for the Bookmarks Button now appears in the Navigation Toolbar. Click on it and drag it to the far left of the Bookmarks Toolbar and OK your way out of the dialog box and you're done. As a bonus I've now got the folder holding most of my RSS subscriptions next to the Bookmarks Button which sort of unifies the user interface rather better.