Unfortunately Firefox doesn't support mask-image group of CSS properties, so we did SVG clip-paths for the examples - they are included in the zip. However, you won't be able to access the SVG if you load the styles from CDN due to XSS limitations - you will need to reference the CSS from the same domain to get the clip-paths in Firefox.
That said, we don't fully support Firefox Mobile, mainly due to its broken flex box implementation.
the Telerik team
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