It is possible the update got corrupted on your PC. I would suggest simply downloading Firefox 15 from www.getfirefox.com and installing it manually for now.

I've had the same problem for several updates - it can't be that it get's corrupted every time and it is a bit annoying having to uninstall and install instead of automatic update. I have recently received messages that something is trying to trick Firefox to install (I can't remember exact wording). Could it be that Firefox update is conflicting with my firewall / virus scan?

Just as a note, you don't have to uninstall Firefox before installing a new version.

It is entirely possible that it's your Firewall/anti-virus interfering, what products do you have?

Then one of the other help pages needs an update. ;-) I followed the instruction there. I'm using ESET Smart Security 5.

Create a new profile as a test to check if your current profile is causing the problems.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be careful not to copy corrupted files.

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

it has something to do with extensions (add ons) disable one bye one and find the one who is responisible for the "not updating" , for example disable the extension or add on "https everywhere"