Compact Borders are no longer supported on Windows
If you've been pointed to this page, it means you're using Mozilla Firefox 135 or newer on Windows, which features a design change that breaks Compact Borders.
As documented on https://github.com/angelbruni/Geckium/issues/419, a change by Mozilla was made in Firefox 135 and onwards that makes the native window borders now permanently show on Windows with absolutely no way to hide them on our side, which completely breaks Compact Borders visually due to Geckium expecting to only have its own custom borders instead.
The change can be traced to this Bugzilla bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1930292
New versions of Geckium on Mozilla Firefox 135 and newer now detect this change and adjust Compact Borders to be square and gapless, as seen in Geckium's predecessor Silverfox, to account for the now forced Firefox window borders.
NOTE: Usage of Firefox is governed by its Terms of Use, while forks are exempt from this.
Note
Mozilla Firefox 128 ESR will be supported until September 16.
Mozilla Firefox ESR releases prior to Mozilla Firefox 140 predate this change and therefore do not have this issue whatsoever, remaining compatible with Compact Borders on Windows - you can get it from Firefox's website.
Another situation is to use an unofficial Firefox fork such as those listed on https://github.com/angelbruni/Geckium/wiki/Compatible-Firefox-forks-with-Native-Windows-Titlebars-support - as long as the fork restores chromemargin, the fork will not have the issue and thus will be fully compatible with Compact Borders.
Finally, you can also switch to Linux or macOS as, unlike Windows, these two alternative Operating Systems never had native window borders in Firefox, making Compact Borders fully compatible regardless of the version of Mozilla Firefox used for Geckium.
This difference also plays a crucial role in allowing Firefox on Linux to have custom borders, such as a full AERO look for Mozilla Firefox when paired with https://gitgud.io/wackyideas/aerothemeplasma.