Why you may have lost access to plugins or extensions in Firefox 21

" ... If you have upgraded your Firefox browser to Firefox 21, the version that got released yesterday, you may have noticed that plugins or extensions that were formerly usable in the browser were not anymore after the upgrade.
"While you may not even mind that, depending on the plugins or extensions that disappeared, some of you may have noticed that they could not make use of functionality on specific websites or the browser anymore due to the change.
"So what happened during the update that caused this? ..."

Source: Ghacks Technology Newsletter
Details  Why you may have lost access to plugins or extensions in Firefox 21