bzugodesign.com | Hugo Velez

OMG! It was what I said about the solution.

The problem:
Firefox automatic downloaded a new version and asked for restart.
Restarted and then I get:

Software Update Failed
One or more files could not be updated. Please make sure all other applications are closed and that you have permission to modify files, and then restart Firefox to try again.

Weird but after a lot of search and a few tries to change file permissions and other things around it , I discovered that YzDock was blocking somehow the update installation.

The Solution:
YzDock options -> Quit.

Open firefox, the update installation is done and voi?la…done.

Edit: This affects both firefox and thunderbird.