Firefox 38 notifies you about add-ons that slow-down the browser.

Mozilla plans to integrate notifications to Firefox that inform users if add-ons have been detected that may slow it down.

The feature, similar to what Microsoft integrated in its Internet Explorer browser years ago, monitors the performance of add-ons in the browser.

If an add-on crosses a certain threshold, the warning is displayed in the Firefox interface.
The warning notification reads "[Add-on Name] might be making [Firefox version] run slowly". Add-on name in this regard is the name of the add-on that is also displayed in Firefox's add-on manager while Firefox version lists the channel (e.g. Nightly).


Three action buttons are displayed in the notification area:

Disable add-on - This turns the add-on off and works similar to manually disabling the add-on in the browser's add-on manager.
Ignore for now - Won't do anything for now. It is likely that warning messages will be displayed again at a later point in time.
Ignore permanently - Don't display warnings anymore and don't do anything.

Firefox seems to display lots of warnings right now which suggests that the threshold may need adjustment before the feature lands in the stable version of the browser.

If you happen to run lots of add-ons, you may be bombarded with notifications in the beginning.