>>834
#navigator-toolbox[iconsize="large"][mode="icons"] #nav-bar #unified-back-forward-button #back-button {
-moz-image-region: rect(0pt, 18px, 18px, 0pt) !important;
padding-left: 4px !important;
}
#navigator-toolbox[iconsize="large"][mode="icons"] #nav-bar #unified-back-forward-button #back-button > .toolbarbutton-icon {
border-radius: 2.5px 0px 0px 2.5px !important;
margin: 1px 2px 1px 1px !important;
padding: 2px 5px !important;
}
#navigator-toolbox[iconsize="large"][mode="icons"] #nav-bar #unified-back-forward-button #back-button:not(:hover) > .toolbarbutton-icon {
box-shadow: 0 0 0 1px rgba(255,255,255,.3) inset,
0 0 0 2px rgba(255,255,255,.1) inset,
0 0 0 1px rgba(0,0,0,.15),
0 1px 0 rgba(0,0,0,.2),
0 1px 1px rgba(0,0,0,.1) !important;
}
#navigator-toolbox[iconsize="large"][mode="icons"] #nav-bar #unified-back-forward-button #forward-button {
mask: none !important;
margin-left: -11px !important;
}
#navigator-toolbox[iconsize="large"][mode="icons"] #nav-bar #unified-back-forward-button #forward-button > .toolbarbutton-icon {
margin: 1px 1px 1px 0px !important;
padding: 2px 5px 2px 12px !important;
}
#navigator-toolbox[iconsize="large"][mode="icons"] #nav-bar #unified-back-forward-button #forward-button {
-moz-image-region: rect(0pt, 36px, 18px, 13pt) !important;
padding-left: 0px !important;
}

14.0.1で使っているのは↑です
紹介いただいたスタイルを試してみましたが、こちらも抉れるようです・・