* And, most annoyingly, the add-on version check and resulting incompatibilities.
That last one actually is kind of a pain, since, as other people have mentioned, not all add-on authors have updated their add-ons to be compatible with 6.0, despite that being a single character change to an XML file for 98% of them. (Fortunately, addons.mozilla.org does this for authors, so it is only the ones hosting the add-ons themselves that are a problem.)
A couple of examples of where Mozilla does make the version number clear:
* The download link
* The downloaded file (Firefox Setup 6.0.exe)
* The new version page that automatically comes up the first time you run the new version (http://www.mozilla.com/en-US/firefox/6.0/whatsnew/)
* And, most annoyingly, the add-on version check and resulting incompatibilities.
That last one actually is kind of a pain, since, as other people have mentioned, not all add-on authors have updated their add-ons to be compatible with 6.0, despite that being a single character change to an XML file for 98% of them. (Fortunately, addons.mozilla.org does this for authors, so it is only the ones hosting the add-ons themselves that are a problem.)