[Affected platforms]:
Platforms: Windows 10

[Steps to reproduce]

1. Launch the Firefox browser with a fresh profile.
2. Close and start Firefox again with the same profile.

[Expected result]
The text from the "Set Firefox as your default browser" modal should not wrap on a second row.

[Actual result]
The text from the "Set Firefox as your default browser" modal wraps on a second row.

Hm, I thought I knew what was happening here but I actually can't reproduce on last night's nightly.

Can you check what build you're on, and what Windows DPI settings you see this at?

I was in the latest Nightly, the build from today, I could only reproduce it on Windows 10 and 7, resolution is 1920 x 1080 with the recommended 100% scaling.

This is the build id: 20210414160838

(In reply to Rares Doghi from comment #2)

I was in the latest Nightly, the build from today, I could only reproduce it on Windows 10 and 7, resolution is 1920 x 1080 with the recommended 100% scaling.

Thanks for the super quick response.

This is gonna be funnn, rounding errors! I can repro with 100% scaling, but not with 150% (the default for my machine). Investigating further...

As @jfkthame pointed out on matrix, 'ch' units are based on the
rendering of the '0' character, and thus their width in CSS 'px'
can vary for fonts that line things up to device pixels, which
can cause pretty big variations if you start multiplying with
large numbers like the dialog width. As a result, the dialog
was 30 CSS pixels narrower at 100dpi than at 150dpi, which
caused the wrapping.

I also noticed we forgot to update the width for checkboxes
and text in dialogs after our font-size changes in bug 1701920,
so I took the opportunity of fixing that here.

https://hg.mozilla.org/mozilla-central/rev/ea0c0f99c53d

This issue is Verified as fixed in our latest Nightly build 89.0a1 (2021-04-16).