For starters, I personally think I'm switching back from Chrome, as Firefox 4 is again a really fast browser. Chrome has its main advantage in speed (and it still retains the supremacy), but Firefox is the most extensible browser and now it has an acceptable speed. Thus I have Firebug (and not Firebug Lite), DownThemAll! and many other extensions again.
Statistics after some days from launch
While I'm writing this, after a week from launch, the download counter for Firefox 4 is at 45 millions and counting. As you can see at glow.mozilla.org, every second there are dozens of downloads. North America and Europe are tied with roughly 15 millions each.
This is the trend for market share between browsers:
As you can see, the enemy of Firefox is not Internet Explorer anymore, but rather Chrome. With this release, Firefox gets inline with Chrome on performance and features.
Firefox 4 will probably the browser used during development for many of us. Currently, it supports the majority of HTML 5 features.
- HTML 5 History, for respecting HTTP paradigm also in web 2.0 applications.
- IndexedDB for harnessing local storage.
- WebM video support and hardware accelerated WebGL, HTML 5 Audio Api.
- CSS 3, <canvas> element and SVG.
- Geolocation, drag and drop, and more...
After Chrome, also Firefox is using automated synchronization between instances of the browser on different machines.
Chrome has a big advantage here: the greater diffusion of Google Accounts. However, I tried the process for setting up Firefox synchronization and it's straightforward (apart from the key which reminds me of program's serial codes): you never leave the same window to perform registration and starting the sync.
There is much talking about OAuth and OpenId today, but they may become less important as more and more browsers save passwords and sync them (Firefox 4 also with end-to-end encryption of these data). After the hassle of registering to a new web application, I usually never remember the password anymore as the browser can do it for me; now it can do it for all my machines.
Already from version 3 Firefox answered to the unique bar of chrome with the Awesome Bar, which you can use not only for entering Urls but also to search the history.
Still, both Firefox 3 and 4 maintain a separate search bar. It's hard to prefer one of the two solutions: Chrome's bar is really fast when you have to search Google (strange, it isn't?) but when I have to enter queries for different search engines, like YouTube or Wiktionary, a separate search box is less confusing and able to remember my last search engine. Chrome implements multiple search with keywords: typing youtube.com searchquery will search in the OpenSearch Api of YouTube. If you want to adopt OpenSearch, two major browsers will support you.
Customization is maybe the most important feature of firefox.
First, with extensions: I remembered not being able to switch to Firefox 4 alpha because of the lack of the Delicious extension. Now Firefox 4 is the stable release and the target for every extension developer.
But also with personas, skins for the browser (which you can actually try in Firefox 4 simply by hovering the persona's button). We'd like to have the control over the graphical design of our web sites and applications, but still we have to deal with with users with different browsers and different graphics: how does your Web 2.0-esque design plays with black, white, or textured browser components? And there are also different Themes that an user can choose: for example, I chose a minimalist theme for my netbook to save screen real estate.