Remembered about this hidden browser.compactmode.show Firefox about:config setting, enabled it in Customize toolbar... and added a few tweaks to my userChrome.css file. Considering the browser is open all day for me, it's worth optimising the environment a little for open tabs. #til #firefox

/* make it easier to find active tab */
.tab-background {
&:is([selected], [multiselected]) {
border-top: 1px solid #444 !important;
}
}

/* thinner pins, even in compact toolbar mode */
.tab-content {
padding: 0 4px !important;
}

/* remove space between tabs */
.tabbrowser-tab {
padding: 0 !important;
}