Why can't Firefox run local .swf files?

Found a workaround here which is based on an Arch thread here. From what I understood, it's just a problem with Firefox not properly supporting a certain mime-type.

Edit /usr/share/mime/packages/freedesktop.org.xml as root and replace the following:

<mime-type type="application/vnd.adobe.flash.movie">

With:

<mime-type type="application/x-shockwave-flash">

And then run:

sudo update-mime-database /usr/share/mime

Got local .swf files working properly now.

Another option is to create ~/.mime.types in the home directory with following content:

application/x-shockwave-flash       swf swfl

This way it will not get overwritten on system upgrade.

If you want you can copy, paste and run this command to do this:

echo 'application/x-shockwave-flash       swf swfl' >> .mime.types

No further commands necessary, not even a browser restart.