[Affected versions]: - 62.0.3, 63.0b12, 64.0a1 (2018-10-08) [Affected platforms]: - win 10x64, Ubuntu 16.04, macOS 10.13 [Steps to reproduce]: 1. Launch Firefox; 2. Access about:preferences#home 3. Click the dropdown for the Homepage and new windows option; 4. Click on Custom URLs... 5. Grab the browser and resize to shrink its width. [Expected result]: - content is properly displayed; - not sure of the proper behavior, but probably the button width should remain the same to contain the text; [Actual result]: - the following buttons are changing width to adjust for resize but text is overflowing: [Use Current Pages], [Use Bookmark...] [Regression range]: - introduced with bug 1417155 [Additional notes]: - attached screenshot with the issue;

This is caused by the buttons having -moz-box-flex (flex=1 attribute) and a defined min-width ( https://searchfox.org/mozilla-central/source/toolkit/themes/osx/global/button.css#12 , https://searchfox.org/mozilla-central/source/toolkit/themes/linux/global/button.css#16, https://searchfox.org/mozilla-central/source/toolkit/themes/windows/global/button.css#16 ), which in XUL means they can be resized as part of flex. There's a trivial fix for this, I'll just put up a patch. While this may have regressed as part of bug 1417155, that's only because this stuff is now in its own column so there's less available width in the first place. I bet in locales with longer labels this would have been reproducible in the old layout, prior to bug 1417155.

Pushed by gijskruitbosch@gmail.com: https://hg.mozilla.org/integration/autoland/rev/59b646e9e899 set min-width to auto for flexed buttons in the prefs, r=jaws

Verified with 64.0a1 (2018-10-21) on macOS_10.14, Ubuntu_16.04, Win_10x64.