CMEX-feel пишет

может подойдет:
[spoiler]@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */

#TabsToolbar .tabbrowser-arrowscrollbox,
#TabsToolbar .arrowscrollbox-scrollbox,
#TabsToolbar .scrollbox-innerbox {
display: block !important;
}
.scrollbutton-up,
.arrowscrollbox-overflow-start-indicator,
.scrollbutton-down,
.arrowscrollbox-overflow-end-indicator {
display: none !important;
}
.tabbrowser-tab {
min-height: var(--tab-min-height) !important;
vertical-align: bottom !important;
}
/* Изменить высоту вкладок */
/* -------------------------- */
tab {
height: 10px !important;
}
.tabbrowser-strip {
height: 12px !important;
}

/*
* Edit this file and copy it as userChrome.css into your
* profile-directory/chrome/
*

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