Firefox has some extensions to the Web Developer part, but it's still not as useful as Firebug from what I can tell. Other than some other minor visual changes that may show up more to Mac users, more of a security update with some CSS/HTML5 additions thrown in.
Thunderbird just seems to have a Search the Web context menu whenever you select text and right click, that doesn't launch an external browser, shows you it inside a new tab which is kinda cool. It also seems to have the Test Pilot addon installed, don't remember that being there before but maybe it was.
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I've put them both on my netbook this morning. Funny thing though...Firefox 10 kept failing to install, forcing me to restart the system and then rolling back to 9.0.1. After this happened 4 times, I uninstalled 9.0.1 through the Control Panel, then used IE to download 10.0 from the Mozilla site.
When this installed and launched, all my tabs, bookmarks, add-ons etc were still there from 9.0.1. How did that happen?
The uninstall won't remove your profile(s). So the new install will see you have them and use them.