When you install or upgrade to Firefox 40 on your computer, you’ll not only notice the new welcome page, but also the new design for the add-ons manager, this was done based on the In-content Preferences style, what you also spot is Add-ons Manager no longer displays the version number for extensions and plugins in the list-view.
Mozilla has intentionally removed this because they felt “the average user doesn’t need the version numbers to manage their add-ons and Firefox takes care of updating for them. The version numbers are still in the tooltips, detail view and about:support”.
Firefox 40: Make Add-ons Manager to display version Information for Extensions and Plugins
You can see this change in Firefox v40 instead of 41.
What are your options to know an add-on information?
You can click ‘More’ link for an extension or plugin.
Or install Classic Theme Restorer add-on, visit its Options > General UI (1), scroll down to the bottom, and check ‘Add-on managers add-on list: show add-on version’ and click OK
Visit Add-ons Manager.