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Mozilla is currently stalled on adding support for SVG favicons because the mimesniff spec does not include a process to match SVG files. Linking their bug https://bugzilla.mozilla.org/show_bug.cgi?id=878922#c3 Assuming it needs to be added to 6.1 and 7.1, though 7.1 is unlikely applicable for this use.
The associated bug has been closed as WONTFIX. If this is an issue that should be addressed, please re-raise it as a GitHub Issue.