swishmax alternative for linux?

I don't know of any direct Swishmax clones/alternatives on Linux.

The closest I can think of is Synfig - a 2D animation program which I believe can export animations as flash/.swf format (amongst many other formats). So synfig might be OK if you only want to create animations.

Blender (although traditionally a 3D CAD/animation type program) can also be used to create 2D animations (along with many other things like compositing, non-linear video editing, game creation with the built-in game engine etc) - But Blender has quite a steep learning curve!

If you want to create more interactive content like games, IMHO the best bet would probably be something like the Adobe Flex SDK which will give you the tools required to compile/build .swf files from AS3 and mxmlc files. But this means that you will be doing everything in code. If you are comfortable writing code, you shouldn't have a problem using the flex SDK. But if you're one of those 'spaghetti-code nested inside clips and all over the timeline' type flash-devs, you probably won't get on well with it!

Also when using the Flex SDK you'll need to either create your graphics in code, programmatically (which requires a bit of maths/geometry/trigonometry knowledge), or you can use another external image editor like Inkscape to create scalable svg GFX or Gimp/Krita for bitmap/raster GFX and then write AS3 code to import (or embed) and manipulate the GFX in your project.

There is also a program called Minibuilder, which can be used as a frontend / IDE for the Flex SDK. It's a free/open source Adobe Flash/AIR based application written in AS3 and is almost like a lightweight version of FlashDevelop. Minibuilder will run on any platform. But it requires you to have Java, the Adobe AIR runtime and the Adobe Flex SDK installed (so not perhaps as lightweight as it could be, that's quite a lot of dependencies!). But it will act as a simple IDE for flash development using AS3... If you can get it running that is. Not sure if that one is still under active development! I took a look at it back in 2010 when it was first released, but I haven't used it in years now. It might still be worth a look!

Another one which comes to mind is Ajax Animator - a cross platform clone of the Flash IDE which works in your browser... Never used it myself, but it's supposed to be like Flash MX 2004.

Those are about the only programs I can think of offhand which come close to swishmax!