If there are 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 disk image (dmg) file to the desktop

• Firefox 7.0.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.

• http://kb.mozillazine.org/Profile_folder_-_Firefox

I have done all that before and even with the new version I am still asked daily via a popup to now update the security of 7.0.1 I hit accept and it just indicates that it is connecting to the update server and never does or completes the download or task