> Is there anything compelling in Firefox 4 to get excited about?
Sure! Here are some things I personally find exciting about FF4. I'm not saying everyone should find them exciting, they are just my personal opinion:
* FF4 is the only web browser to combine a tracing JIT with a method JIT for JavaScript. Some websites run amazingly fast with that. See the mozilla demos site for examples.
* IE9 has great hardware acceleration on Windows (but JS engine is inferior to FF4 and Chrome), while Chrome has a great JS engine (but hardware acceleration on Windows is inferior to FF4 and IE9), while FF4 is the only browser to have both great hardware acceleration on Windows + a great JS engine.
* Firefox is the only major browser that is 100% open source. (Chrome isn't open source, it bundles closed-source Flash and a PDF viewer as well as other stuff. Chromium is open source, but the vast majority of Chrome/Chromium users use Chrome.) Firefox is also developed in the open source way, completely in the open, unlike any other browser (including Chromium, where a lot of development is behind closed doors, then 'dropped' into the open - e.g., CrankShaft).
* Almost the same codebase used on desktop and mobile (mobile FF4 will hit RC very soon too). Opera has a mobile version, and Safari and IE have mobile versions too, but they are quite distinct. Firefox brings the mobile and desktop versions closer than those. Among the benefits is you can view desktop versions of websites in FF4 mobile very well. (Of course you generally want mobile versions of websites - but now and then, you do want a desktop version, especially on a tablet.)