It's a very tough call between Linux and Firefox.
LINUX: Many MAJOR websites use Linux or a derivative. Furthermore, it helps small businesses turn into larger ones-- beyond being free, it's more reliable, stable, secure, and efficient. That's especially beneficial to small businesses, since they can put time & money into development instead of paying for Windows, the better hardware it requires to run decently, constant software updates to avoid being hacked, etc. For a 1 or 2 person operation trying to get off the ground, that can make all the difference between success or failure.
Basically, the Internet as we know it today wouldn't be the same without Linux.
FIREFOX: Not only is it a credible threat to IE's monopoly, but I strongly believe that it's responsible for many of the innovations we've seen over the last year or two. Ajax, despite being an overhyped buzzword (and not even a new technology), has been one of the biggest improvements I've seen since I started browsing websites in 1994. Sure it works in IE, but I think Firefox is why it was able to take off. Here's why:
As anyone else who is in the Web design/programming business knows, Internet Explorer is a quagmire of proprietary functions, directives that are called by standards-compliant names but do not actually obey any known standards, and random bugs. And to add to it, Javascript debugging is essentially impossible, since its debugger provides useless error messages on line numbers that appear to be the output of a random number generator. My favorite IE error: "null is null or not an object".
Basically, there was no way for any developer to know if any cool stuff they did was in compliance with any known standards, or if took advantage of random IE bugs that may or may not exist in the future. And when a client is wanting their Web application ASAP, there's no time for experimentation.
When Firefox came along, developers could try new techniques while being reasonably confident that anything they did was standards-compliant, rather than using proprietary MS crap or taking advantage of a random IE quirk. They could debug their code with a console that actually pointed out errors. It's not hard to see why those abilities made innovation possible.