Firefox 114 introduced a new “feature” in the way it renders non-native menus so that they resemble GTK4 more closely. This has the knock on effect currently of making such menus look a bit odd in Breeze.
There’s a bug report here - https://bugzilla.mozilla.org/show_bug.cgi?id=1837651
This “feature” can be reverted in
about:config by setting