If it is Actionscript 3, you can use: http://www.createjs.com/zoe
If it is Actionscript 2, you run into problems. My hope is that Swiffy will become an open source project in the future. If so, it should be available on github.
Software found:
Adobe Wallaby (install Adobe Air beforehand, not working with AS2 fla-files)
Flabaco (only for banners)
Commercial products:
SWF to HTML5 converter (not tested): http://www.fla-exporter.com/
Adobe Flash Professional CS6 can publish FLA files as HTML5
Adobe’s Animate CC can export FLA files to HTML5, read here
And from the Swiffy site:
Developers who currently create Flash SWF files have several ways to switch to HTML5 including Adobe Animate and Google Web Designer. If you need to play an existing Flash SWF file in your browser alone, you might be able to use Mozilla’s Shumway.
Adobe Animate (free trial)
Mozilla Shumway
Google Web Designer
I had great success using the free tool SWFTool – available at https://sourceforge.net/projects/swftools/