In Firefox, the default font size used for commit messages and diffs is too small (10px). In Chromium, it is a larger, more reasonable size (12px).
This was fixed for the old change screen (see T46895 and T42941) by setting an absolute font size of 9pt in site CSS (9pt / (72 pt/in) * (96 px/in) = 12px), though apparently not for the new change screen, which is now the only option.
This, as well as other CSS that only affected the old change screen (e.g. word wrapping of the commit message), should either be changed to apply to the new change screen or removed from GerritSite.css.