the beta5 is working fairly well, and my favorite plugins like no-script, foxy-proxy and firebug work well. Firebug seems to have some documented minor issues but this does not affect to me.
FF4 has two big visible changes, the tabs on top (like google's chrome) and panorama view for grouping tabs (upper-right corner). This is a nice feature, but when you create several groups I have not found how to cycle between them without the mouse. For me is easier to put tabs in different windows and in my mac I can F10 to see the windows and select one (like in panorama when selecting between groups) but I can also alt-` to cycle between ff4 windows, so I can change swiftly between conceptualy grouped tabs, for example from the journals window to the bloger-mail window when writing an entry in my blog.
What I miss is an option to list all windows and tabs opened and be able to select the wanted one. This is just the opposit of when you start firefox from a stored session and an error occur and gives you the list of all the windows and tabs so you can deselect the unwanted one. This options should be somewhere but I have not discovered yet.