Using open source tooling, we have seen it is quite easy to create favicons to support widely both legacy and modern browsers. You can use Inkscape to create an initial favicon as an SVG. It is an open source alternative to Illustrator and got recent improvements which let it run much smoother than earlier versions. You can also use Inkscape from the Terminal to convert the SVG to PNG files in all the sizes you need. We have seen a basic shell script can help optimize this workflow. You can optimize both the output PNG files and also your original SVG with open source tools. For the PNG files, use OptiPNG. For the SVG, try Scour.