Make sure your Security software is blocking updates as well. If this isn't your computer then you need to ask the owner of the computer to update it.

Thanks very much for your answer. The question is what changed in Firefox that before it was not required any privilege, and the update was done straight. Now to install the update something was changed in the way Firefox executes the update.

Installing Firefox does not seem to require administrator privileges. Using the standard user will give the same result. This is how Firefox developers designed it.

Thanks very much for your answer. I installed Firefox from scratch instead of update the version that I had.