On Firefox 60 on x64 Linux, code in markdown (see screenshot, from https://bitbucket.org/marshallpierce/ktor-csrf/src/master/) renders noticeably smaller than the surrounding normal text, which is understandable given that it's set to use font-size: 12px vs the font-size: 14px used elsewhere. If I adjust its font-size to 14px, it looks the same size as the normal text.

On Chrome 66 on the same OS, rendering appears closer to what macOS Firefox does, which has the text all approximately the same size.

Despite being in the minority, it seems like Firefox/Linux's behavior is actually correct, in that it's displaying 12px text as smaller than 14px...