Firefox 3.5 is released – information about having multiple Firefox versions and web developer extension compatibility

Firefox 3.5 was released yesterday, and it has already reached 5 and a half million of downloads (at the time of writing). Therefore, I thought I’d answer some common questions, especially from a web developer perspective about the new version and which web developer extensions which will work with it.

I have to say that I think Firefox 3.5 is an impressive release, it has gotten even better in so many areas! One very important thing is speed: according to performance God Steve Souders, Firefox 3.5 is the best-performing web browser, followed by Chrome 2, Safari 4, IE 8 and Opera 10.

Don’t throw Firefox 3.0 out – run multiple versions of Firefox side-by-side instead

Even though you’re probably super-eager to replace all old versions of Firefox on your machine, please know that it is completely possible (and very simple) to have multiple installations of Firefox side-by-side; and, have them all running at the same time too, if you want to!

You can download any number of versions of Firefox, and then just install it into different catalogs, or just give each installation a unique name (preferably version number). This is very useful, especially if you are a web developer, to quickly test something in a number of versions of Firefox.

Creating different Firefox profiles

Then, naturally, you might want to have different settings, extensions etc installed depending on what version of Firefox you are running, and the easy way to control that is through different profiles in Firefox. Basically, a profile is where Firefox keeps all your settings, preferences, installed extensions etc.

The way to create a Firefox profile are in the Profile Manager. This is how to open Profile manager on different platforms:

Profile manager in Windows

Open the Windows Start menu and choose the Run option (on Vista, it might not be there – just press Windows key + R in that case). In the run dialog, write firefox -P and press enter/click OK. Choose Create Profile in the dialog and follow the steps.

Profile manager in Mac

Open the Terminal (located under /Applications/Utilities) and type in:/Applications/ -profilemanager. Choose Create Profile in the dialog and follow the steps.

Profile manager in Linux

Open a terminal, use cd to navigate to your Firefox directory and then enter ./firefox -profilemanager. Choose Create Profile in the dialog and follow the steps.

Remember to uncheck the Don’t ask at startup checkbox – that way, for any instance of Firefox you start, you can choose what profile you want to use.

A picture of the Profile Manager for Firefox

New offerings with Firefox 3.5 for web developers

All new features are listed in Firefox 3.5 for developers, but let’s give a brief list of what I find exciting:


  • Support for the audio and video elements.
  • Full support for the offline specification
  • Native drag and drop support

CSS features

  • Supporting @font-face rules for downloadable fonts.
  • Support for CSS media queries, meaning support for media-dependent style sheets.
  • The :before and :after pseudo-classes now fully support any CSS 2.1 variation.
  • Supporting the ch unit.
  • -moz-opacity has been fully removed, and replaced by the standardized opacity property.
  • Support for the -text-shadow property.
  • Support for the word-wrap property.
  • The white-space property now also supports the pre-line value.
  • CSS transforms, via the -moz-transform and the -moz-transform-origin properties.
  • Support for :nth-child.
  • Support for :nth-last-child.
  • Support for :nth-of-type.
  • Support for :nth-last-of-type.
  • Support for :first-of-type.
  • Support for :last-of-type.
  • Support for :only-of-type.
  • Support for -moz-box-shadow.
  • Support for -moz-border-image.
  • Support for -moz-column-rule.
  • Support for -moz-column-rule-width.
  • Support for -moz-column-rule-style.
  • Support for -moz-column-rule-color.
  • Support for -moz-nativehyperlinktext.
  • Support for -moz-window-shadow.
  • Support for -moz-system-metric.
  • Support for -moz-appearance.

DOM features

Support for these:

  • localStorage
  • Using web workers
  • Geolocation API
  • querySelector and querySelectorAll
  • Mouse gesture events
  • The NodeIterator object
  • The MozAfterPaint event
  • The MozMousePixelScroll event

JavaScript features

One of the most exciting features are that of native support for JSON, but there have also been some other improvements with the implementation of JavaScript 1.8.1 (also, don’t miss my JavaScript test suite for features and web browser support).


  • Cross-site access controls for HTTP
  • Progress events for XMLHttpRequest
  • Improved Synchronous XMLHttpRequest support
  • Controlling DNS prefetching

Canvas features

  • HTML 5 text API for canvas elements
  • Shadow effects in a canvas
  • createImageData()
  • moz-opaque attribute

Are web developer extensions ready?

Naturally, the most common thing that hold web developers back from upgrading is if their favorite tool will work with the new version. If we look aside the possibility of running multiple versions of Firefox at the same time, and instead at the most popular Firefox web developer extensions and Firefox 3.5 compatibility, things are looking good!

The best web developer tool in the world does work with Firefox 3.5! The latest version, 1.4, is in a beta state, and as a user and developer of extensions to Firebug, I think it’s not perfect just yet, but it does work well overall.
Web Developer
The Web Developer toolbar works just fine with the new Fox!
HTML Validator
Besides Firebug, this is one of the extensions I have become completely dependent on! And luckily, the new version works just fine! Note: you can only download the Windows version from the Add-ons web site, whereas the Mac and Linux versions are available in the developer’s own web site (what’s up with this?).
Page Speed
Above-mentioned Steve Souders have made sure that his Page Speed extension to Firebug works.

And, if one takes a look at the Web Developer’s Toolbox Collection in the Add-ons web site, you can see that every extension but one is compatible with Firefox 3.5. The only one I have found, that I use, that won’t work with Firefox 3.5 is YSlow.

And really, I don’t know why YSlow isn’t ready for Firefox 3.5. I think extension developers have had quite some time to prepare, and Mozilla have helped out well with their article Updating extensions for Firefox 3.5.

My only guess is that the YSlow people didn’t feel ready to develop for the current beta version of Firebug, and maybe are waiting for a later release. Whatever the reason, I sure hope they release a new version for Firefox 3.5 and Firebug 1,4 soon, because it’s a great extension.

And, naturally, I would like to mention that my extensions all work with Firefox 3.5 as well:

Therefore, don’t worry. At least as a web developer, virtually all vital extensions are ready!


The release of Firefox 3.5 is very exciting, and I’m really happy it’s out now! I should also mention to you Swedes out there, at least those that are Stockholm-based (or willing to travel), there will be a release party later this summer! However, since all of Sweden seem to be on vacation during July, this will probably take place in August. Stay tuned!

And, to end this article, if you’re in the mood, there a pretty Firefox 3.5 wallpaper available as well. 🙂


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