Yeah, I generally do _not_ like programming-oriented ligatures, but the one for todo is sort of interesting.

However, many editors can accomplish the same goal by highlighting or changing the text colour of todos.

Emacs package for that (highlights others like FIXME etc): https://github.com/tarsius/hl-todo

You could make it have the outline thing if you tweak your theme. I wouldn't start installing or hacking your fonts to get this kind of stuff.

Fira Code supports stylistic sets, so you can actually configure whether == and === (and != and !==) do that, and leave weirder ligatures out of the defaults. In VS Code, for example, you would change editor.fontLigatures from true to "'ss08'" to make them still ligate together, but with breaks between the = signs. I use ss06 to make backslashes faded when they're used to escape other characters.