Mozilla introduced benchmark figures for add-ons, which show their impact on Firefox's startup speed.
On average, each add-on you install adds about 10% to Firefox start-up time. For some users that’s mere milliseconds, and for others it may be half a second; it all depends on the hardware and software of each individual. Many add-ons add less than 10%, and unfortunately, there are quite a few add-ons that add more. But based on our real-world performance data, installing 10 add-ons will double Firefox’s start-up time.
What is Mozilla doing about it?
We’re excited to announce a number of initiatives we’re working on to reduce the performance impact of add-ons through developer tools, documention, and user education.
Automated performance testing — Every week, we perform automated performance tests of the top 100 add-ons hosted in our gallery and display the results. Soon, we will automatically scan new versions of all add-ons as they are submitted. In the coming months, we will expand this to include other measures, such as page load time.
Slow performance warnings — In the next two weeks, we’ll begin displaying warnings in our gallery for any add-on that slows Firefox start-up time by 25% or more. We think users deserve to know when an add-on will impact them and want them to make an informed decision to install the add-on. In an upcoming version of Firefox, these warnings will be displayed in the Add-ons Manager as well.
Performance documentation and outreach — We’ve updated our Performance Best Practices and have begun reaching out to developers of slow add-ons and asking them to work on performance. This isn’t limited to add-ons hosted in our gallery; it includes all slow add-ons we can find.
On-demand performance testing — In the coming months, we’ll provide tools for developers to upload an add-on and receive a performance report to test the effectiveness of their fixes on the same machines that officially rate their performance.
Required opt-in for installation — It’s an all-too-common practice of third-party software to install toolbars and other bundled add-ons in your browser without permission. We know that these add-ons account for many of the performance problems reported to us, and users often don’t know how the add-on got there or how to remove it. In an upcoming version of Firefox, third party add-ons will not be installed unless the user explicitly allows the installation in Firefox. We expect this to have a huge impact on Firefox performance, as well as giving users back the control they should have over their add-ons.