Nice workaround. Here's how to set up the file association in Firefox. Run Firefox.
Windows:
Alt (to show pulldowns), Tools / Settings
Mac:
Firefox / Preferences
Click General at the left. Scroll just past halfway down and you will see the attached graphic.
Find the
filterforge type and highlight it. Click the action field. Your choices for filter download that make sense:
Always ask - Lets you pick any version of Filter Forge (that are in the list) to download the filter to. If no versions are in the list "Use other..."
Use other - Change the default program to download the filter to.
Note: FF14 default locations:
Windows - C:\Program Files\Filter Forge 14\Bin
Mac - /Applications/Filter Forge 14

Math meets art meets psychedelia.