Firefox 5 Gets Faster Connections, Up Next: Memory Improvements

Martin Brinkmann
Jun 13, 2011
Updated • Mar 15, 2012

Firefox 5 is a week away from being released as a final version. The browser is expected to be released as final on June 21. When you look at the changelog you will notice quite a few under the hood improvements that have not been talked about yet. HTTP Transactions sorted by CWND is one of those features. Most users probably wouldn't associate a faster browser with that feature in particular, but the explanation on the Bugzilla site might change that.

What really distinguishes different connections to the same server is the size of the sending congestion window (CWND) on the server. If the window is large enough to support the next response document then it can all be transferred (by definition) in 1 RTT.

It basically means that Firefox may load resources faster if connection handling and priorities are changed.

I've done an experiment to show the best case - a link to a 25KB resource off of a page that contains a mixture of small and large content. In both cases the 25KB resource is loaded with an idle persistent connection. In the historic case it reuses a connection that had loaded a small image previously and it takes 3RTT (793ms) to transfer it.. in the case of sorting by cwnd the window is large enough to accommodate the entire resource and it is all complete in 1 RTT (363ms). Cool!

Even better, the worst case scenario is the status quo of Firefox 4. Users who are interested in a longer, more technical explanation, can visit the Bitsup blog for a taste of that.

Firefox 4 transfer

Firefox 5 transfer


The guys over at HTTPWatch have tested the new feature and found the "the performance benefit [to be] substantial".

In other news: Firefox has a bad reputation for excessive memory usage, and related to this slow downs especially on startup or when closing the browser window. While that is certainly not the perception of all Firefox users, many perceive Firefox as a browser that uses to much memory.

The MemShrink project aims to optimize Firefox's memory consumption. The developers list speed, stability and perception as the three core benefits of optimizing the memory usage of the Mozilla Firefox web browser.

The project members will analyze memory leak reports and prioritize them based on numbers of affected users and their default priority.

The developers have created a new website called Are We Slim Yet which tracks the process of cutting down on Firefox's memory usage.

If things go forward as planned, we might see considerable memory footprint reductions in coming versions of the browser.


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  1. Yury G said on September 2, 2020 at 9:32 pm

    Well, the improvements sure didn’t last long. It went downhill after 17esr and after 31.8 became total bloatware. I switched to Chromium44 and IE11, good stuff :-)

  2. Tim Bonham said on June 19, 2011 at 6:00 am

    “While that is certainly not the perception of all Firefox users, many perceive Firefox as a browser that uses to much memory.”


  3. Leslie M said on June 14, 2011 at 2:34 pm

    I am in the unfortunate position that I live in an area where currently the only internet connection is via dial-up. I upgraded to Firefox 4 and noticed that it is much SLOWER than FF3.6.17 – but I read that FF4 was supposed to be much faster. So, my question is will FF5 really be faster or is it simply doing things behind the scenes to appear faster if you have broadband ?

    1. Martin Brinkmann said on June 14, 2011 at 2:43 pm

      Firefox 4 should be faster than Firefox 3. My suggestion for dial up users is Opera, because of Opera Turbo which speeds up web browsing noticeably.

  4. Ghax said on June 14, 2011 at 1:31 am

    Firefox code base have successfully become a pile of shit fuck mess. It’s huge, complex, and unnecessary. It’s like linux now, so open that everyone one to put everything in it. But nothing work well. I suggest they push the reboot button like MS did with IE9.

    1. Cattleya said on June 15, 2011 at 1:33 am

      So much extension have this feature, just install Restartless Restart, Personal Menu, Restart Firefox:
      This: Restart Firefox
      and this:
      These add-on is small and restartless, it does not make Firefox startup slower.

    2. Chomp said on June 14, 2011 at 1:50 pm

      You criticize Firefox and Linux then praise Microsoft and IE. By any chance is this Microsoft’s vast “marketing” resources at work or just a deluded fanboy?

  5. Ahmad said on June 13, 2011 at 9:55 pm

    I want to help Mozilla Team cut down few megs from memory leaks but unable to do so. But I admire there efforts. Thanks for comprehensive article martin. Please cover some more if you have. What about posting info related to Mozilla TI javascript engine, electrolysis, Chrome new v8 improvements etc.

  6. Paul(us) said on June 13, 2011 at 9:54 pm

    Are you aware or i still need the memory fox add on with the mentioned memory leakage optimization project memory shrink or/and need it?

    1. Martin Brinkmann said on June 13, 2011 at 9:58 pm

      It is not really clear what the devs do to reduce the memory footprint.

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