If you have problems with updating or with the permissions then easiest is to download the full version and trash the currently installed version to do a clean install of the new version.

Download a new copy of the Firefox program and save the DMG file to the desktop

• Firefox 5.0.x: http://www.mozilla.com/en-US/firefox/all.html

• Trash the current Firefox application to do a clean (re-)install
• Install the new version that you have downloaded

Your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks and other personal data.

You can maybe re download a fresh new copy of Firefox 5.o, at http://www.mozilla.com/en-US/firefox/fx/, but first before downloading the fresh copy of Firefox 5, delete ALL firefox things on your computer. After your fresh new copy of firefox 5, check if it is the 5th version by pressing 'about Firefox' button located in your firefox button. In the 'about Firefox' , it should have a little number , which locates your version. It should than say 5.0. Therefore that means you have firefox 5.o. But don't be mistakened, that its firefox 4 when its firefox 5 , since Firefox 4 and 5 are similar looking, and Firefox 5 only has minor changes comparing to Firefox 4.

SOLVED. I uninstalled/erased all traces of the previous versions, and 5.0 loaded with no problems. I did however take the advice of the previous post and downloaded a fresh copy of the DMG file. So far so good.. Thanks for the advice.