rustdoc UI: Warning block icon is no longer a circle (on my machine) but a squircle

Reproducer:

//! <div class="warning"></div>

Current output (FF 140.0.2, Linux, rustdoc 1.90.0 nightly 2025-07-30 3048886e5):

Expected output (copied over from https://github.com/rust-lang/rust/pull/106561, only consider the icon not the rest):

I think this is a regression (on my machine) but I can't retroactively find a "good version". I've tried 1.74 (first stable version to include this feature) and 1.75 to no avail (it's not a circle). Maybe it's always been that way on my machine, I don't remember. I did remove the doc/ folder before trying a different version and I did hard-refresh.

The "i" is ⓘ, U+24D8 Circled Latin Small Letter I and I believe it's rendered with font Fire Sans, however I don't know how to confirm that using the FF dev-tools since I can't seem to inspect the computed style of pseudo elements. We do serve a local copy of Fira Sans if not available:

https://github.com/rust-lang/rust/blob/e3ee7f7aea5b45af3b42b5e4713da43876a65ac9/src/librustdoc/html/static/css/rustdoc.css#L73-L80

And for me it should serve that copy since I don't have Fira Sans installed (fc-list | rg Fira has empty output). Now, the font-face rule I've linked above declares the weight to be 400 which seems to check out with what I'm seeing. Then, I've checked how ⓘ (U+24D8) gets rendered per Fira Sans weight (https://fonts.google.com/specimen/Fira+Sans?preview.text=%E2%93%98) and that's what I'm seeing in my browser (image is edited to compact the data):

So whether the i is circled or surrounded by a squircle seems to depend on the weight (for everyone?) and since we pick a weight of 400, it's not a circle (for me). If we picked 300 or 600, it should render as circle.

I'm wondering tho, is it only me who has this issue? Since it renders like a squircle for me even on the introductory version 1.74 I feel like my setup is busted. What?

Start in src/librustdoc/html/static/css/rustdoc.css, especially the font-face rule around lines 73–80, and reproduce the warning block with the supplied Rust snippet in Firefox. Compare the rendered icon across the relevant Fira Sans weights and determine a CSS or font choice that matches the expected circle; done means the warning icon renders consistently as a circle.