Browser add-ons are a great way of bolting on new functionality and customizing the experience to a high degree. But they're also a great way of slowing down browser quite a lot, which is why Mozilla is introducing several new measures to cut down on poorly optimized add-ons for Firefox.

"Firefox performance is extremely important to our users, especially how quickly it starts up and loads websites. Customization is also extremely important, and while most add-ons cause only a tiny performance impact, others can significantly slow down Firefox," Mozilla explains.

"Many users don’t realize add-ons can cause these delays, and that’s why we’re committed to improving performance in a big way," it adds.

From testing, Mozilla has determined that the average add-on adds about 10 percent to the starting time of Firefox. Since people either have no add-ons at all or several of them, the impact is great.

If you have a modern machine, you may not notice the slow down at least until you have quite a few add-ons, but on older computers even a small number is going to have an impact.

This is why Mozilla will undertake several steps to get add-on makers to optimize their creations. One way is by automatically testing the most popular add-ons in the Mozilla online gallery and displaying the results.

The worst offender, Firebug, for example, adds 75 percent to Firefox's loading time. Of course, some less popular add-ons may and most likey do add even more time.

Mozilla plans to extend the automatic testing to each new or updated add-on that is submitted. There are also plans to add more metrics such as the impact on page load times.

The add-ons that add more than 25 percent to the startup time will be marked as such, starting in the next couple of weeks, notifying users that they're about to significantly slow down their browser.

But Mozilla is also helping developers speed up their add-ons. There are updated Performance Best Practices, for better documentation, and Mozilla is also reaching out to the creators of slow add-ons to notify them.

Developers will also be able to test their add-ons to see how they perform before they submit it, during the development process.

Finally, one of the most interesting change, and the one that will have the biggest impact, is that it won't be possible to install add-ons behind the scenes without the users' permission.

This is done by plenty of apps and it can quickly fill the browsers of unsuspecting users with toolbars all manner of similar garbage. A future version of Firefox, Mozilla doesn't specify which, won't allow this behavior and the user will have to approve all installations.