They are still the only browser I know, which has actual useful chrome like changing the stylesheet, is CUA compliant and behaves and feel like a native GTK+ app (now-a-days only after restoring the OS window bar and enabling the menubar).

They also have useful keyboard behaviour and provide both a search and a URL bar, which makes it effortless to search locally and perform additional refinery searches while hunting down something, because you can change the search term without returning to the search website. Searching via the search engines portal is also often slower than via the search bar on crappy connections. Their search provider integration is also great (not sure how other browsers are in this regard) which makes opening a Wikipedia or MDN page about a specific topic a single action, without needing to look at a search result list.

There Profile Manager is also a breeze (not the new crap), it allows to open any URL in any Profile by clicking on any link in another program.

The extension system and the advanced configuration is also quite good.