Every WordPress "Send to Kindle" button that I've tried, whether on my blog, or others, exhibits this same behavior:
When using Opera, I click the Send to Kindle button and a new tab opens with the following error message:
There is a problem.
Your browser does not work with Send to Kindle. Please click below to get the latest version.
Firefox (<- which links to http://www.mozilla.org/firefox)
I'm sorry if the exclusion of Opera support was mentioned elsewhere. If it isn't, it probably should be!
Opera 12.14 (newest) on OS 10.8.3.