All text is italicized on Firefox on Linux

[melvyn2]

Describe The Bug
All text on the Web UI is italicized, even when it shouldn't be.

Steps To Reproduce

1. Create a clean Firefox profile (from about:profiles)
2. Visit https://demo.jellyfin.org/stable/web/#/login.html
3. Observe that all text is italicized

Expected Behavior
The UI text is not italicized.

Logs

Screenshots

System (please complete the following information):

• Platform: NixOS Unstable
• Browser: Firefox 134.0.1
• Jellyfin Version: 10.10.3

Additional Context
Also seen on Fedora Workstation 41 on Firefox of the same version, but not on PopOS with Firefox of the same version: https://forum.jellyfin.org/t-all-text-on-webui-is-in-italics-on-1-browser

Removing the font-family rule of the html selector shows the text in an un-italicized serif font instead. This rule:

html {
  font-family: Noto Sans,Noto Sans HK,Noto Sans JP,Noto Sans KR,Noto Sans SC,Noto Sans TC,sans-serif;
}

However, applying the same rule to other websites does not italicize their text.

Removing the following font directive also removes the italicization while keeping the usual sans-serif font:

@font-face {
  font-display:swap;
  font-family:Noto Sans;
  font-style:normal;
  font-weight:400;
  src:local("Noto Sans"),
  local("Noto Sans Regular"),
  url(c417ee867416d52e5187.woff2) format("woff2");
  unicode-range:u+00??,
  u+0131,
  u+0152-0153,
  u+02bb-02bc,
  u+02c6,
  u+02da,
  u+02dc,
  u+2000-206f,
  u+2074,
  u+20ac,
  u+2122,
  u+2191,
  u+2193,
  u+2212,
  u+2215,
  u+feff,
  u+fffd
}

as does removing the two local sources (forcing it to use the server-provided font).

[skittlesvampir]

I can confirm, same issue on Firefox on Linux, but removing the custom CSS does not fix it for me...

[srcrist]

I tested this and am unable to reproduce. This may be an issue with the distro or the browser itself, but it does not seem to be a Jellyfin bug.

[srcrist]

Tested with Ubuntu 24.04 LTS desktop and the snap install of Firefox.

[kudos]

I'm seeing the same issue on Fedora Silverblue 41.

[kudos]

I think this thread is related https://discussion.fedoraproject.org/t/font-on-websites/142269/7

[kudos]

Here's how Lichess fixed the issue lichess-org/lila#16806

And the Mozilla bug tracker https://bugzilla.mozilla.org/show_bug.cgi?id=1941820