fontforge VS birdfont

You should be able to convert .fon files using https://github.com/fontforge/fontforge .

Fontforge doesn't currently have a way to vertically center glyphs - in the script I'm currently working with, all glyphs should be vertically centered, so this is very inconvenient. I saw that there was one open issue on this very topic - but the last activity was in 2016, so I don't think it'll get added anytime soon. Instead, I decided to spend some hours on tripping over my skills to read the documentation.

You are but, as of today, reliably, only via the GUI, and therefore, one by one, not in batches via scripting due to a bug[1]. That is the case at least for Type 1 Mac fonts.

[1]: https://github.com/fontforge/fontforge/issues/4668

FON files are Windows bitmap fonts, aren't they? You could convert them to ttf using Fontforge.

All I what I saw are premium software. The only software I found at the lowest price is (FontBird)[https://birdfont.org/] = $10. But I don't know, I downloaded the free version (which doesn't support color fonts) and it seems to me that the software is not maintained well or glitchy!

Try Birdfont - https://birdfont.org/

FontForge is the best free font editor right now, although there are some others in development. A great place to start learning is to read the Design with FontForge ebook, which will quickly teach you a lot of the basics that can be applied no matter which font editing software you're using. If you're struggling with FontForge, you can also try BirdFont which is a bit simpler.

Besides, for adding the ligatures I use Birdfont for Linux.

If you don't find any other Birdfont users here, you could also try their Facebook group or post an issue on their Github repo.

More seriously, not really. Well maybe you can try Birdfont but usually FontForge is regarded as the most complete free editor. Birdfont looks more simple so it might work for you, I don’t know much about it.