Firefox 18 has support as well, but it's going to take a while before it goes stable
The CSS Flexbox is getting a lot of love lately. It's only supported in a stable browser by Chrome, as of Chrome 21, but Mozilla and now Opera have started working on the latest draft spec of the Flexbox.The latest Opera snapshot now boasts support for the feature, the third browser to work on implementing the latest iteration of the specifications.
What's interesting though is that, even though Mozilla added support for the CSS3 Flexbox in Firefox earlier, Opera may get it to "market" way faster than Firefox.
The latest snapshots are of Opera 12.10, which is already in beta. The stable release is just around the corner and the Flexbox has every chance of being in it.
The mobile Opera 12.1 also adds support for the updated syntax. But Mozilla has only added it to Firefox 18, which should be landing in the Aurora channel any time now, so it's going to take 12 more weeks before reaching the stable channel.
If you're interested in Opera's implementation of CSS3 Flexbox and about the feature itself, Opera has some documentation waiting for you.