This article only applies if you installed Firefox manually (without using your distribution's package manager).After updating Firefox to version 50.0 my Firebug opens the default developer tools. I have always preferred Firebug as my default debugging tool. Though at some point this preference will be removed.
Open a terminal and run the following commands: W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/adobe-flashplugin_20160407.1tar.gz' couldn't be accessed by user '_apt'.
Ubuntu should have an updater (don't remember exact name) that checks for and asks you about installing (or optionally automatically installs) newer packages/software.
The firefox on my ubuntu requires me to update Flash plugin and follow the link I get to the page which ask me to download the latest Firefox package. It is the new version, but it is in an independent folder and if I want to use it, I need to open the folder and click on the Firefox executable.
Mozilla updates its Firefox web browser frequently, pushing down security fixes and new features that all of its users should enable.
Therefore, it's in your best interest to make sure your browser is set to automatically install these revisions.