Slow Performing Firefox Add-ons Revisited

Martin Brinkmann
May 6, 2011
Updated • Dec 15, 2012
Firefox, Firefox add-ons

Last month Mozilla published a list of slow performing add-ons for the Firefox web browser. The findings back then were that add-ons increases the startup time of the web browser by ten percent on average. That's a lot, considering that five add-ons would on average increase the browser's start-up time by 50%.

The basic idea was to inform users and developers alike about the top add-ons that slow down the start-up of the
Firefox web browser. Slow Performance data has been added to the Mozilla Firefox Add-on Gallery.

The biggest offenders back then slowed down the browser by 74%. A Firefox user who'd install the top 5 add-ons of the list would slow down the start of Firefox by almost 300%.

The situation looks as grim one month later. The top five add-ons now slow down the browser by more than 350%. Some add-ons that have been previously on the list are not found in the top 10 anymore, but most add-ons that have been in the top 10 back then are still there, many with slower start-up times than before.

slow firefox startup

Mozilla has modified the top listing slightly. Only a top 9 list is displayed on the Slow Performing Add-ons page on Mozilla. Back then it was possible to load a top 50 list, which does not appear to be possible anymore.

Operating system filters have been added to the page which have not been there before, making it now possible to display the slowest add-ons for Fedora, Mac OS X, Windows 7 or Windows XP instead of a mixed listing.

slow firefox addons

If you look at those listings you will notice that only the add-ons of the top 9 listing are displayed. The start-up performance on the other hand differs widely. The worst offenders of all lists are SimilarWeb and Personas Plus under Windows 7. They slow down the startup time of Firefox by 223% and 202%. Compare that to the 25% or less add-on start-up time on the tested other operating systems. These numbers may be flawed, and it is very likely that they are considering the huge difference between Windows 7 and all other tested operating systems. While a flawed test is the most likely explanation, it could on the other hand be an extension specific problem instead. Only re-tests will tell.


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  1. ipadwise said on August 7, 2011 at 9:15 am

    The best way to speed up Firefox is to switch to Google Chrome. I love everything about Firefox except its speed. Chrome is way faster, so I put up with the current lack of extensions like an ad blocker etc. Mozilla really need to address the speed issue with Firefox and they need to do it now.

  2. Alvin Koffman said on May 10, 2011 at 7:40 pm

    That’s why I went to Chrome

  3. Simon B. said on May 6, 2011 at 6:01 pm

    Well, the numbers are % = relative. The most likely explanation is that Firefox starts very much quicker in absolute numbers on the Windows 7 testing machine. Could be simple as that the Win7 machine uses SSD disks and the others use SATA, or could be due to Win7 preloading doing an excellent job with the firefox binary but missing out completely on the extensions, while perhaps other operating systems do preloading (something with similar effect) of both firefox and its extensions.

  4. AnonCoward said on May 6, 2011 at 5:18 pm

    Are startup times really that important? I start the browser and it usually sits in the task bar the whole time I’m using the computer, so somehow I don’t feel it makes that much of a difference if it takes 6 rather than 2 seconds for it to start.

    What I think is more important is the browser’s reliability, stability, and responsiveness WHEN IT’S RUNNING. Faster startup times, better JS rendering, HW acceleration that doesn’t even work properly on most video cards and makes the text blurry – all that stuff is irrelevant. The real problem is that the latest version of Firefox is even slower than its predecessors. With or without addons, it runs like an overfed pig with one of its legs missing and its UI is just as responsive as a heavily sedated sloth.

    I really Mozilla would focus more on making the browser snappier / lighter / faster and less on increasing the version number and trying to make it look more like Chrome.

    1. boris said on May 6, 2011 at 5:48 pm

      For people like you and me few seconds of startup time is irrelevant. But I do help some people with their computer and they shutdown Firefox almost immediately after they were browsing for five minutes. And then when they start Firefox again and if it does not show up in five seconds they start complaining that Internet is slow.

  5. Ashley Pearson said on May 6, 2011 at 4:00 pm

    They should improve as the browser does!

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