He wasn't forced out for his beliefs. He was forced out for donating money to those trying to make it a law that everyone--even folks who disagreed with him--had to live according to his beliefs, for not changing his mind--or at least acknowledging that this law forcing everyone to live according to his beliefs hurt real people--and, because that has turned out not to be such a popular thing to do, especially in his industry, for being a potential financial and media drain on the company that'd just made him their public face.
The thing about free speech (in the broad sense--by this point everyone is aware that this was not government action and is therefore not a 1st A issue) is that it does not protect you from other people using their free speech to criticize what you said using yours. He spoke his mind (money being speech, n'all), a lot of folks used their speech to disagree with him and seek remedy, and the free market had it's say, as well...
Those writing posts and deleting their FireFox browsers and other Mozilla products over this guy's resignation are not doing anything different that the folks who support marriage equality were doing a week or more ago because he was hired. I did not participate in the boycotts against the guy, and I don't personally believe he should've been forced out either, but my opinion, like those who're all up in arms now, did not prevail. They saw the landscape and made a corporate decision.
There's nothing wrong with folks who're passionate about an issue voting with their wallets and their feet, whether it's the marriage equality folks for the last few weeks or the traditional marriage supporters in the last few days. Sometimes it actually works.