Trainspotting: Firefox 39

Trainspotting is a series of articles highlighting features in the lastest version of Firefox. A new version of Firefox is shipped every six weeks – we at Mozilla call this pattern “release trains.”

A new version of Firefox is here, and with it come some great improvements and additions to the Web platform and developer tools. This post will call out a few highlights.

For a full list of changes and additions, take a look at the Firefox 39 release notes.

DevTools Love

The Firefox Developer Tools are constantly getting better. We’re listening to developers on UserVoice, and using their feedback to make tools that are more powerful and easier to use. One requested feature was the ability to re-order elements in the Inspector:

Editing and tweaking CSS Animations is easier than ever – Firefox 39 lets developers pause, restart, slow down, and preview new timings without having to switch applications.

Menu of animation easing presets in the Inspector

CSS Scroll Snap Points

CSS Scroll Snap Points in action

CSS Scroll Snap Points let web developers instruct the browser to smoothly snap element scrolling to specific points along an axis, creating smoother, easier to interact with interfaces with fewer lines of code.

Improvements to Firefox on Mac OS X

Firefox gets some Mac- specific improvements and updates in version 39:

  • Project Silk enabled – Improves scrolling and animation performance by more closely timing painting with hardware vsync. Read more about Project Silk.
  • Unicode 8.0 skin tone emoji – Fixed a bug in the rendering of skin tone modifiers for emoji.
  • Dashed line performance – Rendering of dotted and dashed lines is vastly improved. Check out the fixed bug for more information.

Service Workers Progress

Firefox’s implementation of the Service Workers API continues – fetch is enabled for workers and is now generally available to web content, and the Cache and CacheStorage are now available behind a flag.

There’s lots more changes and improvements in Firefox 39 – check out the Developer Release Notes for developer-oriented changes or the full list of bugs fixed in this release. Enjoy!


  1. Christoph

    The CSS Scroll Snap Points did not work in IE 11, even though the article claims: (works in Firefox 39+ and IE 10+)

    July 1st, 2015 at 00:18

    1. Potch

      I’ve updated the demo with the -ms- prefixes- can you try again?

      July 1st, 2015 at 17:30

      1. Christoph

        Now it works, but only if I click on the arrows next to the scroll bar. If I drag the scroll bar itself, I can scroll to a position between the two “pages” and it remains there.

        July 1st, 2015 at 23:52

      2. Sirquini

        Could you add the overflow-y: hidden; to the non-snap example, it look ugly without it. ;)

        July 2nd, 2015 at 20:20

  2. John Doe

    Will Project Silk be enabled on Windows as well?

    July 2nd, 2015 at 02:05

    1. Monessem

      yes, will be enabled on windows on Firefox 40 according to Release Note

      July 2nd, 2015 at 18:42

  3. Andrew Kavanagh

    Updated to the current FF 39, now it crashes every time I open it. Any idea when a new patch/update may be scheduled?

    July 2nd, 2015 at 14:53

  4. Tim Hamilton

    Ditto John Doe: Is there any ETA on Silk landing for Windows? Cause we need it…bad. Motion quality in FF looks shabby on win compared to just about every other browser…including IE.

    July 4th, 2015 at 11:16

    1. Marcel

      There is one point in the v40 beta releasenotes: “Smoother animation and scrolling with hardware vsync (Windows only)” ( ). So i think we will see this in about 5 weeks on windows.

      July 5th, 2015 at 08:50

  5. Luke

    I wonder, am I the only one seeing this bug with tinyscrollbar and Firefox-39 on Ubuntu? I can’t actually click their scrollbars and scroll, but mousewheel works.

    July 6th, 2015 at 07:45

  6. Andrew Kavanagh

    Hello, I made the mistake of commenting on this thread & now I can Not remove myself from getting emails of others comments.
    I have tried every combination of choices with Manage your subscriptions but I keep on receiving new comments from this thread.
    How can I be removed?

    July 6th, 2015 at 21:12

    1. Havi Hoffman [Editor]

      @Andrew Kavanagh My apologies (2x) — that is most annoying AND unexpected. I believe I have just now ‘suspended’ comment notifications on your behalf, although I don’t understand why you don’t have the ability to do it for yourself. I will investigate further.

      Also, apologies for the crashiness you encountered early on with FF39. A more stable release went out over the weekend. Thanks!

      July 7th, 2015 at 11:09

      1. Andrew Kavanagh

        Thanks, Havi!

        July 7th, 2015 at 14:17

  7. Randy

    Firefox 39 (including any recent weekend release) is still repeatedly crashing on my two different Ubuntu 14.04 LTS PCs, in multiple separate Firefox profiles on those PCs.

    Everything was fine prior to Firefox 39. I receive Firefox with all the other updates, through the Software Updater. To my knowledge, I’m not using any PPA that contains any special version of Firefox, beyond the main Ubuntu PPA (which contains its own special version of Firefox for reasons that are not at all obvious).

    Even with all available updates installed as of this second, Firefox still crashes within minutes of launch.

    July 14th, 2015 at 20:14

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