Firefox 15 Released August 28 – New Features and Improvements

Firefox 15 has been released today (Tuesday 28th August). It includes a variety of new features and improvements – plus one that some users may not like…

New Features

  • ‘Tabs on Top’ option removed from new installs of Firefox – this will annoy some people who prefer to keep tabs on bottom… You used to be able to tick/untick the ‘Tabs on Top’ option from Firefox \ Options or by right clicking on the toolbar – it was easy to enable/disable it in this way.

Firefox 15 removes the ‘Tabs on Top’ option from both places – if you (re)install it from scratch. If you upgrade to version 15 it will keep your existing settings and menu options intact. However, if you install Firefox 15 from scratch it will default to Tabs on Top – with no user interface way to change it.

Fortunately it is still possible to change it via about:config preferences as detailed below:

–  Open Firefox and type about:config in the address bar and press Enter. Click the “I’ll be careful I Promise” warning button (if it appears) to reveal a long list of preferences.

–  Double click the browser.tabs.onTop preference to change it from ‘true’ (the default) to ‘false’ – this moves tabs to the bottom again i.e. below the toolbars.


No ‘Tabs on Top’ option above ‘Toolbar Layout’

Tip: You can scroll down the list of preferences but the quickest way to find it is to type part of the preference name e.g. onTop into the Search box at the top of the config page

[Double clicking the preference again will change it back to ‘true’ and re-enable tabs on top].

  • Start-up performance improvements – Leverages the Windows maintenance service to improve the browser’s start-up time.
  • Memory Improvements – Reduces memory leaks caused by some add-ons.
  • Silent update: Background updates – Moves the bulk of the update process to the current session so that when the user restarts, there’s not much left to do to complete the update and it appears as if no interruption actually happened. This only applies to Mac/Linux – for Windows, the Mozilla Maintenance Service will do this.
  • Accessibility Improvements – Accessibility support for Mac improved.
  • In Content Preferences – The Preferences (Options) window has been moved into the content area i.e. appears in a new tab instead of a separate Options window.

The goal is to eliminate Firefox’s separate management windows in favor of in-content designs. This setting is disabled by default in Firefox 15 but you can enable it right now by following my guide here.

  • ‘Don’t Load tabs until selected’ – This menu option has been moved from the ‘General’ tab to the ‘Tabs’ tab in the Options window as shown below – it does make more sense to have it there:

‘Don’t load tabs until selected’ – now on Tabs menu

Technical (under the hood) Changes in Firefox 15

  • DEVELOPER – Responsive Design View. Allows developers to quickly emulate different screen sizes to see how their designs react to the change.
  • DEVELOPER – Detailed element layout view. The purpose of the Layout tool is to help the user see detailed information about the final dimensions of a page element.
  • DEVELOPER – Debugger. New JavaScript debugger.
  • DEVELOPER – The CSS word-break property has been implemented.
  • HTML5 – Native support for the Opus audio codec added.
  • HTML5 – The <source> element now supports the media attribute.
  • HTML5 – The <audio> and <video> elements now support the played attribute.

Changes Not Implemented

Incremental garbage collection (GC) – the change to enable incremental GC by default was pulled from Firefox 15 due to some outstanding issues. The idea of incremental GC is that games and other dynamic content on the web will have more tolerable pauses, and in some cases will go from having pauses that users notice to not having pauses that users notice.

Although this feature is included in Firefox 14 and 15, it is still disabled by default and not officially supported due to known issues – it is likely to be implemented in the next version. If you really can’t wait you can enable it right now (recommended for advanced users only) by following my guide here – disable it again if you have any problems.

Support for SPDY networking protocol v3 – support for the latest version of SPDY to improve speed and responsiveness. This has been delayed to the next version.


There are a lot of changes in Firefox 15 – in particular, start-up performance and memory improvements are welcome changes.

Technical behind the scenes features help keep it up to date with the latest web standards.

The removal of the ‘Tabs on Top’ option from new installations may annoy some people – it seems to be of more benefit to Mozilla designers than users… Yes, it can be changed via about:config but it’s a pain to do if you need to reinstall Firefox or install it on a new computer – the old menu option was far simpler.

As usual, this new version will be available to users from August 28th over several days (to avoid overloading Mozilla’s servers) – don’t worry if you check for updates (via Firefox Menu \ Help \ About Firefox \ Check For Updates) and find there are none available for you yet, just try again a day or two later.

4 Responses to: "Firefox 15 Released August 28 – New Features and Improvements"

  1. Lian says:

    Thank you so much! I’ve been searching forever for a solution. The new Firefox annoys me so much but my gmail keeping telling me it’s not compatible with old versions of Firefox so i stupidly upgraded. Also, do you know if I can change the page so the browser is not so transparent? It’s so distracting.

    Again, thanks so much for posting this.

    • Roy says:

      Sorry but I’ve no idea what you mean by a transparent page or browser?

      • Lian says:

        Hi, I mean the top of Firefox above the bookmark toolbar is semi-transparent with anywhere that has words having a lighter gray colour background, I find this annoying but don’t know how to change it.
        Actually I find Firefox less convenient than before and wants to change to chrome, except I don’t want to start anew and I need my remembered cookies and passwords >_<

      • Roy says:

        Still not sure but if you mean the menu bar (file, edit etc), you can remove it – View \ Toolbars \ untick Menu Bar to use the new Firefox dropdown menu. Or disable Aero theme in Vista/7 – it’s Aero that makes the menu bar semi transparent.

        If you want to switch to chrome see – can migrate history, passwords, bookmarks across automatically. You could just install Chrome and migrate but keep Firefox as well in case you want to go back to it – no need to uninstall