I use Comics Sans, just for the look on people's faces when they realise.
More seriously, I use Comic Neue, which isn't an ideal font for coding in some languages because it lacks a glyph for "`", which is rather useful in Markdown, and some variants of Bourne Shell and Python. I switched, though, to using a proportional font years ago and never looked back. Comic Neue, unlike it's Sans cousin, is a reasonably well designed sans font.
I've looked at Fira Code, and besides the fixed-font thing, the ligatures on operators give me the wibbles. Ligatures on variable names, though, wouldn't worry me at all.
Proportional fonts are great. The textual parts of the language compress visually much better - this also allows the developer to use more width for identifiers. It's much faster to read, too, and you're less likely to misread identifiers as well. The resultant code tends to have better-named identifiers, too, I find, and the comments... Well, there's absolutely no comparison between reading a block of text in monospace and a block in proportional font.
The only problem comes when some project I'm working on has an arbitrary line length in characters. I can never get along with those, since with a proportional font they're impossible to judge and largely irrelevant anyway.
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