Other things were mentioned already, so not to repeat those: On Ubuntu in particular (but on other Linux distros, too), Firefox 4 has a more platform-appropria

Other things were mentioned already, so not to repeat those: On Ubuntu in particular (but on other Linux distros, too), Firefox 4 has a more platform-appropriate appearance and feel than the other cross-platform major browsers (Chrome and Opera). For Web authors, there are more polished HTML5 form features to get excited about. For HTML5 feature involves new elements or attributes in markup, the chances of the features present in Firefox being polished is higher than the chances of the features present in Chrome being polished even though it might casually appear from html5test.com that Chrome support a larger number of these features. html5test.com tests if new APIs appear to be present--not if all the default user interaction works right or if the appearance of a feature is pretty across platforms (not just Mac and Windows but Linux, too). The location bar history search in Firefox is remarkably more useful than in Chrome, because Firefox matches on more things than just the prefix of the URL (though Chrome has better search coming up and already available in about:flags). Well, actually, syncing browsing history and passwords across browser instances in such a way that the data in encrypted and decrypted on your devices only is well worth repeating although it was mentioned already. (Syncing history and passwords is immensely more useful than just syncing bookmarks or sending over individual URLs, but history and passwords are the most sensitive browser data, so end-to-end crypto between the user's devices is important.) (Disclosure: I'm consulting for Mozilla. Disclaimer: Speaking only for myself, though.)