userChrome.css in the Photon style for Firefox 93+.

userChrome.css

/*** Tighten up drop-down/context/popup menu spacing (8 Sep 2021) ***/

menupopup:not(.in-menulist) > menuitem, 
menupopup:not(.in-menulist) > menu {
  padding-block: 4px !important; /* reduce to 3px, 2px, 1px or 0px as needed */ 
  min-height: unset !important; /* v92.0 - for padding below 4px */
}
:root {
  --arrowpanel-menuitem-padding: 4px 8px !important;
}

/*** Proton Tabs Tweaks ***/

/* Adjust tab corner shape, optionally remove space below tabs */

#tabbrowser-tabs {
    --user-tab-rounding: 0px;
}

.tab-background {
    border-radius: var(--user-tab-rounding) var(--user-tab-rounding) 0px 0px !important; /* Connected */
    margin-block: 1px 0 !important; /* Connected */
}
#scrollbutton-up, #scrollbutton-down { /* 6/10/2021 */
    border-top-width: 1px !important;
    border-bottom-width: 0 !important;
}

/* 1/16/2022 Tone down the Fx96 tab border with add-on themes in certain fallback situations */
.tab-background:is([selected], [multiselected]):-moz-lwtheme {
    --lwt-tabs-border-color: rgba(0, 0, 0, 0.5) !important;
    border-bottom-color: transparent !important;
}
[brighttext="true"] .tab-background:is([selected], [multiselected]):-moz-lwtheme {
    --lwt-tabs-border-color: rgba(255, 255, 255, 0.5) !important;
    border-bottom-color: transparent !important;
}

/* Container color bar visibility */
.tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
    margin: 0px max(calc(var(--user-tab-rounding) - 3px), 0px) !important;
}

/* Override Normal Density height to Compact Density height (plus room for context-line) only for tabs 1/16/2022 */

#TabsToolbar, #tabbrowser-tabs {
    --tab-min-height: 31px !important;
}
    /* Tweak for covering a line at the bottom of the active tab on some themes 8/11/2021 */
#main-window[sizemode="normal"] #toolbar-menubar[autohide="true"] + #TabsToolbar, 
#main-window[sizemode="normal"] #toolbar-menubar[autohide="true"] + #TabsToolbar #tabbrowser-tabs {
    --tab-min-height: 32px !important;
}
#scrollbutton-up,
#scrollbutton-down {
    border-top-width: 0 !important;
    border-bottom-width: 0 !important;
}

    /* [Connected Tabs] Set a max height based on min-height plus margin-block: 1px 0 */
#TabsToolbar, #TabsToolbar > hbox, #TabsToolbar-customization-target, #tabbrowser-arrowscrollbox  {
    max-height: calc(var(--tab-min-height) + 1px) !important;
}
    /* [Connected Tabs] Adjust padding around icons on buttons to avoid crushed images */
#TabsToolbar-customization-target toolbarbutton > .toolbarbutton-icon, 
#TabsToolbar-customization-target .toolbarbutton-text, 
#TabsToolbar-customization-target .toolbarbutton-badge-stack,
#scrollbutton-up,#scrollbutton-down {
    padding-top: 8px !important;
    padding-bottom: 7px !important;
}

/* Inactive tabs: Outline style */

.tab-background {
    --context-line-margin-adjust: 4px; /* 1/16/2022 */
    --context-line-active-adjust: 0px; /* 1/16/2022 */
}
.tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-background {
    border: 1px solid rgba(0, 0, 0, .20) !important;
    /* To pick up a theme background tab separator color, un-comment the following line: */
    /* border: 1px solid var(--lwt-background-tab-separator-color, rgba(0, 0, 0, .20)) !important; */
    border-bottom-color: transparent !important;
}
    /* For dark backgrounds */
[brighttext="true"] .tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-background {
    border: 1px solid var(--lwt-selected-tab-background-color, rgba(255, 255, 255, .20)) !important;
    /* To pick up a theme background tab separator color, un-comment the following line: */
    /* border: 1px solid var(--lwt-background-tab-separator-color, var(--lwt-selected-tab-background-color, rgba(255, 255, 255, .20))) !important; */
    border-bottom-color: transparent !important;
}
/* Remove extra padding between tabs */
.tabbrowser-tab:not(:first-of-type) {
    padding-left: 0 !important;
}

/* Move container color bar to bottom of tab - updated 1/16/2022 */

.tab-background {
    --context-line-margin-adjust: 3px; /* 1/16/2022 */
    --context-line-active-adjust: 1px; /* 1/16/2022 */
}
.tabbrowser-tab[usercontextid]:is([selected=true], [visuallyselected="true"], [multiselected]) .tab-background {
    border-top: 1px solid transparent; /* 1/16/2022 Default border for consistency */
}

.tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
    margin-top: calc(var(--tab-min-height) - var(--context-line-margin-adjust) - 2px) !important;
}
.tabbrowser-tab[usercontextid]:is([selected=true], [visuallyselected="true"], [multiselected])  > .tab-stack > .tab-background > .tab-context-line {
    margin-top: calc(var(--tab-min-height) - var(--context-line-margin-adjust) - var(--context-line-active-adjust) - 2px) !important;
}

/* Emulate blue tab line from Photon 57-88 UI */

/* Set size and position of the bar relative to background */
.tab-background::before {
  content: "";
  display: block;
  height: 2px;
  margin: -1px var(--user-tab-rounding) 0;
}
/* Set color for active tab */
.tabbrowser-tab[selected] .tab-background::before {
  background-color: #ffffff;
}
/* Set color for other selected tabs */
.tabbrowser-tab[multiselected]:not([selected]) .tab-background::before {
  background-color: color-mix(in srgb, #ffffff 66%, transparent);
}
/* Set color for hovering on non-active/non-selected tabs */
.tabbrowser-tab:not([selected], [multiselected]):hover .tab-background::before {
  background-color: color-mix(in srgb, currentColor 33%, transparent);
  transform: none;
}
/* [Optional] Animation when hovering non-active/non-selected tabs */
/*
.tabbrowser-tab:not([selected], [multiselected]) .tab-background::before {
  transform: scaleX(.8);
  transition: transform 100ms linear;
}
*/

/*** Audio Playing / Mute Button side-by-side when sound is playing ***/

#TabsToolbar {
    --user-mute-button-height: 20px;  /* default size is 12px, site icon is 16px */
}

/* Move the mute/unmute button to the right and enlarge it */
.tab-icon-overlay:not([pinned], [sharing], [crashed]):is([soundplaying], [muted]) {
    width: var(--user-mute-button-height) !important;
    height: var(--user-mute-button-height) !important;
    margin-left: calc(var(--user-mute-button-height) / 2 + 2px) !important; /* pushes icon to the right */
    margin-right: 2px !important; /* closes up some space before the text */
    padding: 0 !important; /* allows icon to expand to full size */
}

/* Move the site icon to the left a bit and adjust position */
.tab-icon-stack:not([pinned], [sharing], [crashed]):is([soundplaying], [muted]) > :not(.tab-icon-overlay) {
    margin-left: -4px !important; /* pushes icon to the left */
    margin-top: calc((var(--user-mute-button-height) - 16px) / 2) !important;  /* keep site icon reasonably positioned */
}

/* Override the rules for hover/not hover visibility */
   /* for mute button */
.tabbrowser-tab:not(:hover) .tab-icon-overlay:not([pinned], [sharing], [crashed]):is([soundplaying], [muted]), 
   /* for site icon */
.tabbrowser-tab:hover .tab-icon-stack:not([pinned], [sharing], [crashed]):is([soundplaying], [muted]) > :not(.tab-icon-overlay), 
   /* for site icon with Compact density */
:root[uidensity="compact"] .tab-icon-stack:not([pinned], [sharing], [crashed]):is([soundplaying], [muted]) > :not(.tab-icon-overlay) {
    opacity: 1 !important; /* overrides full transparency with full opacity */
}

/* Color the icon on hover for confirmation or avoidance */
.tab-icon-overlay:not([pinned], [sharing], [crashed]):is([soundplaying], [muted]):hover {
    fill: green !important;
}
[lwthemetextcolor="bright"] .tab-icon-overlay:not([pinned], [sharing], [crashed]):is([soundplaying], [muted]):hover {
    fill: lightgreen !important; /* for dark themes */
}

/* Tweak Options as of 1/12/2022; Generated Mon Aug 01 2022 19:29:59 GMT+1000 (Australian Eastern Standard Time) */