For extensions, I like the minesweeper add-on. If microsoft has contributed anything to the world, it's minesweeper, and the firefox version (I think you have to google it, I don't know if it's on the official site anymore) allows you to play square, hexagonal, and best of all, with multiple flags per tile. I enjoy it even more than the expert level on the main windows minesweeper. What I don't like is that it doesn't record times, but oh well.
I also like the ability to scan any web page that has a textinput and set it as one of the search engines in the searchbar. If you use the google toolbar for searching, it's like that, but you can use any search without going to the webpage. Amazon, wikipedia, dictionary.com, ebay, and IMDB are set by default, but I've added in the leo german dictionary, too.
I like the ease of skinning with themes, though as yet, it seems Opera had cooler themes.
FlashGot is a popular extension for automating downloading all of the pictures or other content on a site (basically like wget in linux), and is fairly user friendly. I don't know about dload managers in other browsers.
I like StumbleUpon a lot. In fact, it's the best thing to happen to the internet since Google. If I'm looking for something, I use google. If I want to find something I didn't even know I was looking for I use stumble. I think IE has a version, but it works flawlessly in FF either way.
There is an extension for BibleGateway.com that allows you to switch translations/versions and lookup from the toolbar. There are a number of webpage editing extensions, too. I think they've got an integrated Tidy, some css tools, colormaps, a tool to view as it looks in different browsers (including IE), and loads of other little tools ranging from useless to helpful.
Finally, the css support and transparent .png's have been supported for years, nearly 5 with Opera. Sorry, but there is no excuse for the heel-dragging IE has done by failing to support standards Microsoft agreed with the W3C to promote back in 2000.
IE7 will have tabbed browsing. It will have inline search from a toolbar (though I'm interested to see if it's very customizable beyond a few popular and proprietary services). It will have (after being forced) better CSS and .png support. But why should I wait a month, 2 months, 6 months for it to get out of beta development when I can have all these features NOW!? Where will FF be when IE7 is released? How long will it take IE to catch up with those features? Another 5 years? No thanks.
And I wouldn't be surprised if MS bundles software that promotes their Digital Rights Management. "We're sorry, but you haven't paid for the right to look at that website. You'll have to pay 1$ to microsoft so that we can give the site a penny for you to access it." Yes I'm exaggerating, but until IE becomes a cutting edge tool pushing the boundaries of features, compatibility, and ease of use over any other browser, why should I bother?
. I started using firefox, and i'm growing to like it. The tabbed browsing is a big sell for me though.... i HATE when ie clusters together on the taskbar 