First, the good news. Firefox 4 is fast. And, it includes some new features that are useful, like Sync, which will keep your history, passwords, and bookmarks synchronized across all your computers and even your smartphone.

In version 4, the Firefox developers made significant changes to the look of the browser. I suppose this is neither good nor bad, but just takes some getting used to. It’s a jolt, though. They’ve hidden the menu bar unless you dig around and turn it back on. They’ve put the tabs at the top of the window, and they’ve crammed all the menu items into one tiny button “for easier access.”

The bad news, for those of us who use Firefox add-ons, is that version 4 breaks a lot of them. Some add-ons work fine in the new version, but others do not, at least not yet. When you upgrade Firefox automatically looks for updates to your add-ons and installs the ones it finds. If it doesn’t find them, Firefox lets you know they’re broken. It’s up to you to find updates or replacements for those. That’s not a big deal for many users, but for some of us who rely on add-ons to help with the work we do, it IS a big deal.

Full article: Firfox 4 The Good The Bad The Ugly