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.