This article explains why the Help menu item Check for Updates... and software update options are disabled on some systems. For general information, see Update Firefox to the latest version.
Package management on Linux
Firefox's software update feature can be disabled on some Linux systems because the distribution itself handles upgrading Firefox. To update Firefox on these systems, you'll need to use your distribution's package management utility (for example, Synaptic Package Manager, Adept Manager, or Yum).
Many distributions will only provide security and stability updates to Firefox. They will, for example, let you go from 126.96.36.199 to 188.8.131.52 but not to 2.0. To make a major upgrade, you will have to install Firefox outside of your package manager. See Install Firefox on Linux.
If your WindowsMac OSoperating system account doesn't have permission to change the Firefox installation directory, updating Firefox will not work, so Check for Updates... is disabled. To get Firefox updated, log in to your computer as a user with sufficient permissions, for example an administrative account. Check for Updates... should then be enabled.