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 8.0.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 if you uninstall and (re)install Firefox.

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

I have done what you suggested, i downloaded firefox 8.0 and saved it on desktop. i have deleted firefox 3.o. can you tell me what steps i must take to install firefox 8.0, I am not a real wiz on doing this

See:

• http://kb.mozillazine.org/Installing_Firefox#Mac_OS_X
• https://support.mozilla.com/kb/Installing+Firefox+on+Mac