Tested with:

All the latest firefox versions, including the old certificate page design.

Tested on:

Windows 10
Ubuntu 18.04

Steps:

1. Launch the browser and go to https://wrong.host.badssl.com
2. Click Advanced and resize the browser windows to the minimum width.

Actual result:

The buttons from the advanced section are not the same size.

Expected result:

The buttons from the advanced section should have the same size, as the buttons above, from the main page (see the screenshot attached).

Note:

This is also reproducible on the old design of the certificate page.

There's this code which sets the button to 100% when the window is shrinked:
https://searchfox.org/mozilla-central/rev/36904ac58d2528fc59f640db57cc9429103368d3/toolkit/themes/shared/aboutHttpsOnlyError.css#38-43
but it doesn't work because its parent is not set to 100%:
https://hg.mozilla.org/mozilla-central/log/eacd6f08df5e4f6a216ff58ae294d8bc1e22b31c/browser/base/content/certerror/aboutNetError.xhtml
Unless I'm missing something, the parent can just be removed with the rest of the usages of it changed to the button itself:
https://searchfox.org/mozilla-central/search?q=exceptionDialogButtonContainer&path=&case=false&regexp=false