Just a super rudimentary script that I made which hooks CairoSVG up to Pillow because apparently Firefox doesn't support SVG-formatted browserAction images:

import cairosvg.surface
from PIL import Image
from io import BytesIO
import sys

def main(input=sys.stdin.buffer, output=sys.stdout.buffer):
	svg2ico(url=input, write_to=output)

def svg2ico(*args, resolutions={96, 48, 32, 24, 16}, write_to=None, **kwargs):
	t = cairosvg.surface.Tree(*args, **kwargs)
	ims = []
	for res in resolutions:
		cairosvg.surface.PNGSurface(t, f:=BytesIO(), res * 2,
		  output_width=res, output_height=res).finish()
	im = ims.pop(0)
	# Fix for Windows XP and MATE Desktop:
	# add bitmap_format="bmp" to the following command
	# - requires Pillow>=8.3.0
	# - makes the resulting image files larger
	# * https://github.com/python-pillow/Pillow/issues/2512
	# * https://github.com/mate-desktop/eom/issues/310
	im.save(f:=write_to or BytesIO(), format='ICO',
	  append_images=ims, sizes=[im.size for im in ims])
	if not write_to:
		return f.get_value()

if __name__ == '__main__':

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