That was a great article by Wired! We decided to write about them because they are a partly French-based company, and our news site covers French startups, tech and innovation.
Technically, Mozilla Denmark is the highest level subsididary in Europe, so all offices in Europe are offices of that Mozilla Denmark (it says it on the Mozilla Paris office, it's pretty funny) - but it's much more metaphorical than official. It's a matter of where will European Mozillians go to meet up, work together, etc. I don't think London has that appeal; most non-UK Europeans don't consider London to really be Europe - hell, Londoners don't consider themselves to be european.
Actually, I honestly originally switched because I thought ctrl+shift+n was a smarter hotkey to open a private navigation window than ctrl+shift+p, as it is in Firefox. After that, Firefox just didn't do anything to win me back. It's small, but there 99% identical anyway - i's like Chimps and Humans.
Perhaps its integration within the browser, or marketing - I'm not sure. But again, it's not a matter of assessing each feature and comparing the two, because that's what developers do. It's a matter of appealling to the non-developer community using the internet (much larger than the developer community, unfortunately).
Honestly, it's just not the same. I know Mozilla agrees, because my friend is in charge of bringing out a better "App store" for Firefox. It's not enough to appeal to the developer crowd, who's willing to search hard for what they want - you have to do the Apple and Chrome thing, and appeal to people who are not technophiles, and just want a browser with cool tools.
La Defense is, and has been in the past, home to some of the largest European companies. In fashion, you have L'Oreal, LVMH (the company that owns Louis Vutton, among other notables), as well as some of France's more classic tech companies: Bull, SFR, Orange, Capgemini, etc.