Convenience isn't always a good thing. I know I find auto-updating programs and operating systems annoying -- I'd rather apply patches on a case-by-case basis and preferably not in the middle of other tasks. Browsers are one type of application that has long supported additional functionality via add-ons or extensions and many have some sort of background updating system that is not always desirable to have active.
Fortunately, gHacks' Martin Brinkmann has written a guide on how to not only disable add-on / extension updates, but monitor them as well (if you leave updating on).
For Firefox users, the process of disabling automatic updates for add-ons is straightforward; the functionality can be turned on and off on an individual basis or browser-wide.
Chrome, however, requires one to dig into each extension's manifest file (in JSON format) and direct the update URL for the extension to your own system, effectively shutting it down. On top of this, if you do manually update the extension, you'll have to go back in and make this change again.
Check out the full guide via the link below.