Regular Fira Sans has visual issues on Firefox on Linux

[mhasadi78]

I wanted to use Fira Sans for my general system font and it looked quite fantastic in every aspect except below situation. When Firefox gives me a message about any kind of failure, the title looks like this:

I tried other fonts like Noto Sans and there was no problem at all:

I don't know if it's just me or this is a popular issue. I thought notifying you from existence of such thing can be my only move here.
Thanks in advance.

Here is my specs:
Arch Linux (Kernel 5.12.14)
DE: KDE Plasma 5.22.3
WM: KWin
Graphic Platform: X11

P.S. I installed this font with pacman package manager from the community repository package: ttf-fira-sans

[jfkthame]

See https://bugzilla.mozilla.org/show_bug.cgi?id=1715946.

[fitojb]

The Firefox error page no longer uses ridiculously thin fonts, so this issue can be closed.