You should be able to run all the way up to Firefox 16. Any versions after that you won't be able to install due to the age of your computer.

Try the following:

1. Update your Mac. Run Software Update until there are no more updates, restarting your computer as need be.
2. Go to www.getfirefox.com and download and install Firefox 16.

If it fails, give me the exact warning message you receive.

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 15.0.1: http://www.mozilla.org/en-US/firefox/all.html

• Trash the current Firefox application (e.g. open the Applications folder in the Finder and drag the Firefox application to the Trash) to do a clean (re-)install
• Install the new version that you have downloaded
• https://support.mozilla.org/kb/Installing+Firefox+on+Mac

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

SOLVED! Many thanks.