Thanks to Electrolysis project Firefox with e10s enabled started to warn users when it detects add-ons slowing the browser, the user at that time will see a notification at the bottom (check the screenshot below) that an add-on (with that name) might be making Firefox run slowly— for instance, in the screenshot, you can see it as Tab Data— offers the options in the notification to disable, ignore for that time or ignore permanently.

add-on might be making Firefox run slowly notification

Fix: Enable E10s (multi-process) (disabled: An accessibility tool is active)

When you click on ‘disable’ button, you’ll be prompted to restart Firefox to do so, if you click on ‘ignore for now’, that notification might appear again later and if you consider ‘ignore permanently’ even though that add-on is slowing Firefox that notification won’t be shown again in the future.

Follow this bug: ‘Notify user of add-ons that are slowing their browser down significantly’ and read this blog post from Mozilla’s Brad Lassey for more information.

Note: You need to be using the latest Firefox Nightly with e10s enabled to see this.