Angry Birds Opera

Angry Birds Opera was a free version of Angry Birds available for PCs using the Opera web browser. The game was available at the following site: opera.angrybirds.com. However, Rovio didn't get time to release it so it's considered a cancelled version of the game.

The only screenshot taken to the public, it is in the domain opera.angrybirds.com but because it says BETA instead of a Opera or familliar look is probably a placeholder for it or either made for another purpose.

Why it wasn't released to the public
Angry Birds Opera was actually a planned version of Angry Birds. Unfortunately, Rovio didn't get enough time to release the website and game but they show a screenshot of the testing website.

Or because opera was a competitor of chrome, another web game.

Episodes

 * Poached Eggs (The main episode of Angry Birds and appears in other web games, possibly added.)
 * Note: It might be possible that an Opera version of Chrome Dimension existed, altough there is nothing to confirm its existence.

Release Date
The planned release date was 2011 but Rovio didn't release it because of time and work on other projects.

Taken from Angry Birds Wiki