The best alternatives to Fontello are Fontself, Norde Source 1.0, and IconMaker. If these 3 options don't work for you, we've listed a few more alternatives below.
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Create custom fonts in minutes with Fontself Maker, a powerful font maker easy to use and made for all creatives. Fontself Maker is an extension for Illustrator and Photoshop CC to quickly turn any lettering into OpenType fonts.
Norde Source detects all the different colors in your icon set and lets you change a particular color once for the whole set. Along with organizing, searching, resizing and more.
Pictogram makes it easy to use custom app icons on your Mac. It even preserves your custom icons after your apps update themselves. Requires macOS 11.0 or newer.