Do version numbers matter? It appears that they do. With Google's Chrome well into double figures, we'll probably have Chrome 99 by Christmas - and that means Firefox's more sober numbering system runs the risk of making Mozilla's browser look old.
Hence Firefox 5, which doesn't actually have very many new things compared to Firefox 4 and which will shortly be followed by Firefox 6.
Firefox 6 release date is September 2011. What do we get in the meantime?
The big list of what's new in Firefox 5
There's a new button that tells websites not to track you, and Firefox now supports CSS animations.
However, for end users there's very little to see here - and even the CSS animation support appears to be incomplete: when we tested .net magazine's nifty animated 404 page, which uses CSS animation and works just fine in Chrome, it stayed stationary on our iMac.
STUCK: CSS animation means the covers move in Chrome - but they stayed firmly in their place in Firefox
FIrefox 5 user interface
Like its predecessor, the Firefox 5 user interface doesn't quite work on the Mac - it isn't as Apple-y as Safari or as pretty as Chrome, and even applying themes doesn't help much - but it's a lovely thing on Windows.
Whichever version you use, you get the ability to turn open tabs into App Tabs, which keeps key web-based services such as Gmail or Twitter visible without taking up too much space, and you can organise tabs into Tab Groups, which is handy if you're researching something involving multiple topics.
Annoyingly, the only way to save your Tab Groups is by shutting down Firefox; unless you've disabled Firefox's session restore, your groups will be intact when you open the browser again. It'd be much better if you could save groups as you do bookmarks: while you can save a tab group using Firefox's Bookmarks > Bookmark All Tabs option, which only saves the currently selected group. A Save Tab Groups option would be handy here.
As before, Firefox has an address bar and a search box, although if you don't type a URL the address bar takes you to a web search anyway.
EMPTY BOX: Does Firefox really need a separate search box? If it isn't a URL or in your history, the Address Bar searches anyway
The new button that tells websites not to track you
Firefox is the first multi-platform to support the new Do Not Track system, which tells websites not to snoop on you (it's in Safari and Internet Explorer too). It's an online equivalent of the Telephone Preference Service, which stops firms cold-calling you at home: with Do Not Track enabled, your browser tells advertisers and other firms that they shouldn't follow you around the internet or use behavioural targeting when they blast you with ads. The information is sent in the form of an HTTP header every time you request data from the web.
Do Not Track is a good idea, not least because the alternative is having to set opt-out cookies for every online snooper, which would take an eternity. However, as with the Telephone Preference Service, Do Not Track relies on firms playing fair. If a firm decides not to respect Do Not Track requests, there's not much you can do about it.
DO NOT TRACK: Well, that was easy: implementing Do Not Track is a matter of clicking on a single tick box