@pepelsbey

1. Open "about:support"
2. Find "Profile Folder"
3. Open this and create there folder "chrome" (yes, chrome) with file "userContent.css"
4. Write specific CSS:

@-moz-document url-prefix("chrome://devtools/content/") {
.devtools-monospace {
font-family: "JetBrainsMono-Regular", "JetBrains Mono", monospace !important;
}

.inspector-panel #markup-view {
font-family: "JetBrainsMono-Regular", "JetBrains Mono", monospace !important;
}
}