When editing a post, the font in the edit box with source code for the post is proportional:
This does not happen when posting new questions or answers:
Tested with the latest Firefox and Safari on macOS High Sierra.
Fixed now, sorry it took so long. We're currently working on making our CSS more maintainable, but during work like that, ten years1 of technical front-end debt sometimes come back to bite you in the rear.
1 Yes, Stack Overflow is coming up on its tenth anniversary this year!