As hinted at the plugin description some website developer knowledge is needed to dynamically add the 'download' attribute to anchor tags.

Open up the browser console and enter:

$$('a[href*=".webm"]').forEach(s => s.setAttribute('download', s.getAttribute('href')))

Above command line will:
- find all anchor tags on a website which contain the '.webm' extension in their 'href' attribute
- then iterate over found anchor tags
- and set the 'download' attribute to each anchor using the value from the 'href' attribute

This makes the toolbar button appear everyone was searching for.
Adjust code to your needs.