Hello,

Do update only in the Firefox Oficial Page:

• Mozilla Firefox

In the last days a lot of people said about a pop up that ask to you do update Firefox if this are your case, i think better you scann your PC and start Firefox in Safe Mode, sometimes this popup come from addons or malwares.

• Firefox in Safe Mode
• Troubleshoot Firefox Issues Caused by Malware

Hello gjensse, if exist a problem with update or permissions then the better way is to download and install the new version.

1. Download a copy of the latest firefox from http://www.mozilla.org/en-US/firefox/all.html

2. Trash the current Firefox application to do a clean install.

3. Install the version that you have downloaded.

Do not select to remove your personal data, your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks or other personal data.

see also: Installing Firefox on Mac

thank you