The longer something's in beta, the less interesting it becomes. Beta 1 is greeted with oohs of excitement, beta 2 with hums of interest, beta 6 has everyone stifling yawns and beta 12 wonders where everybody's gone.

So it's nice to see that Mozilla has finally decided to stop faffing and release Firefox 4 as a final version. There's a new user interface, dramatically improved tab management, excellent sync and, of course, promises of vastly improved performance.

With Microsoft back in business and Google chucking out new builds of Chrome every ten seconds, the two-year-old Firefox 3.x was beginning to look rather long in the tooth, and there's no doubt that Firefox 4 puts Mozilla right back in the game. However, it's a different game than before. Firefox is no longer targeting a sitting duck; it's battling eagles.

Is Firefox 4 an eagle?

Firefox 4 looks very different. Tabs are on top of the screen now, and you can pin tabs to the Tab Bar as "app tabs", little icons that you can use to tuck always-open sites such as your webmail out of the way. In a nice touch, any pinned App Tabs glow when there's new content.

Firefox 4

NEW UI: The new interface is very pretty, very polished and a vast improvement over previous versions' UI

Firefox 4

MAC UI: Is it just us, or does the Firefox interface look a little bit odd on the Mac?

Firefox 4

APP TABS: A genuinely brilliant idea, App Tabs can reduce running web apps such as mail or RSS to little icons

The orange Firefox button on the toolbar provides quick access to menu items, and there are Home and Bookmarks buttons over on the right. One of the best features is Tab Groups, a useful idea that enables you to organise open tabs into groups.

What's great about that is you can then view just one group at a time, so if you've got a whole bunch of tabs open for work stuff and another bunch for fun, you can easily and quickly switch between the two.

Firefox 4

TAB GROUPS: These make it easy to break tabs into different kinds, for example by separating business and pleasure

If you're running Windows 7 on a touchscreen device you can pinch, zoom and rotate things with your fingers.

Add-ons, themes, stability and standards

The Add-ons manager makes it easy to find new extensions and themes, and its bright interface makes it clear what's running and what's been disabled. It includes a nifty introduction page, which recommends interesting Add-ons that you might not know about, and it's perfectly user-friendly.

Browsing for Add-ons is rather like browsing the iTunes App Store, which we mean in a nice way, and no browser's extensions library can compete with the range of goodies available for Mozilla's browser. Quantity doesn't necessarily mean quality, of course, but the reviews and ratings make it easy to avoid the duffers and find the gems.

Firefox 4

ADD-ONS: The redesigned add-ons manager is very beginner-friendly. It's more like iTunes than an options screen

Firefox 4 also boasts some excellent synchronisation features. It doesn't just sync your bookmarks across devices; it takes your history and even your currently open tabs. If you're constantly moving from machine to machine you'll love this feature.

Firefox 4

SYNC: Firefox's sync features are excellent, transferring bookmarks and even open tabs between devices

Firefox achieves an impressive 97/100 Acid3 score, supports CSS3 features such as transitions for CSS-based animation and uses OpenType for web typography, and the new HTML5 parser supports crucial features for web apps such as dragging and dropping in and out of apps.

Firefox 4

HTML 5: Firefox's HTML video favours Google's WebM and the open source Ogg Theora. Rivals Safari and IE plumped for H.264

New crash protection should finally stop errant Flash Players from bringing down your browser, but everything runs in a single session – Chrome and IE give tabs individual processes.

Performance and benchmarks

Every browser maker promises that their latest version is the fastest version yet, and Mozilla is no exception. There's substance to the claims, however: the final release is significantly faster than the earlier betas, and dramatically faster than Firefox 3.6.

That's partly due to a new JavaScript engine, JägerMonkey, which promises faster performance, and it's partly due to the hardware acceleration on Windows and OS X.

So how fast is it? In Mozilla's own favoured benchmarks, Kraken, Firefox took 9,224ms compared to IE9's 19,136ms. Chrome was slightly faster at 8,794ms.

In the SunSpider JavaScript benchmarks Firefox was slower than IE but faster than Chrome – the browsers achieved 308.5ms, 288.8ms and 346.0ms respectively – while in Google's V8 benchmarks, where bigger numbers are better, Firefox was in the middle with a score of 3,269 compared to Chrome's 7,101 and IE's 2,053.

Firefox 4

BENCHMARK: This is the fastest Firefox yet, but IE is slightly faster in the SunSpider benchmarks

In the real world, Firefox loaded pages, YouTube clips and so on as quickly as Chrome and more quickly than IE9.

We liked

Firefox 4 is noticeably prettier and considerably faster than its predecessors. Long startup times have been banished, and the clunky interface has been swapped for something much more subtle. The tab management is fantastic.

We disliked

The interface isn't quite as polished as rivals, and the Mac version just doesn't look right. Where other browsers split tabs into multiple processes, on Windows Firefox is a single-process application.


Firefox 4 is a vast improvement, and if you're a power user there's lots here to tempt you: App Tabs are superb, Tab Groups are useful and the enormous add-ons library covers every conceivable eventuality.

Being able to sync open tabs is handy, and encryption keeps your synchronised data safe. However, for everyday browsing there's not much to differentiate Firefox from its rivals.

The best Firefox yet? Undoubtedly. Good enough that other browser users will rush to switch? Perhaps not.