[Affected versions]:

• Firefox Release 74.0 Build ID 20200309095159
• Firefox Beta 75.0b7 Build ID 20200322132212
• Firefox Nightly 76.0a1 Build ID 20200323092419

[Affected Platforms]:

• Windows 10
• Mac OS 10.15.3
• Ubuntu 18.04 x64

[Prerequisites]:

• Have quantum bar activated (set “browser.urlbar.update1” to “true”);

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Load about:newtab page.
3. Horizontally resize the browser to minimum.
4. Observe the Address Bar.

[Expected result]:

• There are no layout issues.

[Actual result]:

• Default text from Address Bar is cut.

[Notes]:

• Issue is also [reproducible on Windows 10] using Firefox Release 74.0 (no matter if Quantumbar is on or off) and not reproducible on Beta 75 and Nightly 76.

This is how placeholder text generally overflows and there's no sane way to prevent this, considering that the string width is variable (depends on the locale and system font size).

(In reply to Dão Gottwald [::dao] from comment #1)

This is how placeholder text generally overflows and there's no sane way to prevent this, considering that the string width is variable (depends on the locale and system font size).

Thanks for looking over this so quickly!
As I mentioned in the initial comment, the issue is not visible in Nightly 76 and Beta 75 while using Windows 10, so, I was thinking that might be a way to fix this for Mac and Linux as well.

Attaching a screenshot from Nightly 76 - Win 10 - where the issue is fixed.

(In reply to Valentina Virlics [:vvalentina], Ecosystem QA from comment #2)

As I mentioned in the initial comment, the issue is not visible in Nightly 76 and Beta 75 while using Windows 10, so, I was thinking that might be a way to fix this for Mac and Linux as well.

afaict that's just because you can't make the window narrow enough on Windows to force it to happen, presumably due to differences in font, native window frames/widgets, etc.