A common complaint about Firefox is that it’s just too slow, particularly upon launching. And after touting the speed and enhanced performance in the browser’s most recent release, Mozilla wants to make it clear to users: if Firefox is slow, it’s probably your add-ons.

Of course, the wide variety of add-ons and the ability to customize your browser to your liking is one of the things that has long made Firefox an appealing choice. But in a post on its blog, Mozilla reveals some stats from performance-testing. It found that for every add-on you install, you’re adding about 10% to the amount of time it takes for Firefox to start-up.

According to its automated performance testing, Mozilla has found that in fact some of the most popular add-ons are among the worst culprits. Firebug, for example, ranks #2, slowing launch speed by 74%. Adblock Plus makes the browser launch 21% slower, and the StumbleUpon add-on, 19%.

Firefox says it’s going to start labeling add-ons that are drags on performance, displaying warnings on those that slow start-up time by more than 25%. In an upcoming version of Firefox, these warnings will be displayed in the Add-ons Manager as well. It also says that in this new version, third party add-ons will not be installed unless the user explicitly allows the installation in Firefox. This will likely cut back on the unwanted toolbars and bundled add-ons that people inadvertently install.

Firefox says it’s also reaching out to developers, to help them make sure their code is optimized and that they are building add-ons with best practices in mind.

For users, it’s always a good idea to disable – or better yet uninstall – add-ons that you aren’t using.