can't do that. SWF is flash. You can export an applet, which runs in a web browser just like SWF (but uses Java instead of Flash). Export Applet button is right there in processing.

you might be more interested in the possibilities of processingJS though : http://processingjs.org/

How does that ProcessingJS work???? It's not an applet, what is it?

to keep it short. processing code run inside the browser without the need of an applet. As they say on the website

"You write code using the Processing language, include it in your web page, and Processing.js does the rest. It's not magic, but almost."

there is also a tool for easy export. https://github.com/fjenett/processingjstool
but it will be included in the new processing IDE though anyway.

No magic in ProcessingJS, just advanced JavaScript usage... It is a kind of Java interpreter, converting the Processing code to JS, and reproducing the graphics library with the Canvas technology.
Somehow, a similar thing could be done in Flash (even more as ActionScript is originally inspired from JS) but I am not aware (or I don't remember) of such thing.

Thanks everybody for these answers.