I recently had to create an extension for as many browsers as I could, which ended up being Chrome, Safari, and Firefox. The easiest - by far - of these platforms was Chrome.

The Safari distribution channel is a ghost town: http://extensions.apple.com/

Most Safari extension makers choose instead to just host the raw executable on their site, but that makes pushing updates a pain. See http://textbooksplease.com (visit in Safari).

Safari does, however, have a really nice extension builder which is useful if you don't know XML, I guess. And when you get down to the nuts and bolts (at least as far as I ventured), the Safari Extension API is pretty similar to Chrome's.

Firefox has been a royal pain. They have a "7-day review process" for all submitted extensions, and a tiered structure of reviews (preliminary and final) which translates to not being able to push updates for weeks at a time. Super frustrating when I can push changes to Chrome in about two minutes and know it will get pushed to everyone who already has it installed in a timely manner. I also found their API to be poorer documented than Chrome's.

I'm terrified of IE.