HTML self-closing tag `/>` issue

Using a regular 1:1 DPI screen on Windows 10. I have a custom style sheet that replaces many <code> and such styles with Fira Code, e.g. on GitHub:

.diff-editor *, code, pre {
    font-family: "Fira Code" !important;
    font-size: 10pt;
    font-variant-ligatures: common-ligatures;
    line-height: 1.3 !important;
}

One issue that happens can be seen in the README here: https://github.com/vuejs/jsx-next where the closing slashes of the tags are almost invisible:

Computed styles (according to Firefox)

box-sizing: border-box;
color: rgb(0, 92, 197);
font-family: "Fira Code";
font-size: 13.6px;
font-variant-ligatures: common-ligatures;
line-height: 17.6833px;
overflow-wrap: normal;
word-break: normal

The ligature works in normal sizes elsewhere and is not cut off (so it might perhaps be a Firefox issue)?

Forcing font-size: 20px; resolves the issue, but is a bit too large for my taste:

Reproduce the README rendering from https://github.com/vuejs/jsx-next in Firefox on Windows 10 with the reported custom CSS and Fira Code at 13.6px, then compare it with the 20px result. No project file or test is identified; the issue is done only when the rendering cause and an actionable fix or upstream report are established.