Styles to revert Firefox Proton tabs appearance, where the active tab is attached to the page below. Active tab is indicated by a line at the top, and background tabs are separated by a thin stroke.

userChrome.css

/*** Proton Tabs Tweaks ***/
/* Modified from https://www.userchrome.org/firefox-89-styling-proton-ui.html#tabstyler 
   to previous tabs appearance with colour line over selected tab
*/

/* 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;
    margin-block: 1px 0 !important;
}

/* for >= v102 */
.tab-background[selected]:-moz-lwtheme {
    outline: none !important;
}
.tab-background:is([selected], [multiselected]):-moz-lwtheme {
    border-color: var(--lwt-tab-line-color, var(--lwt-tabs-border-color, currentColor));
    border-style: solid;
}

#scrollbutton-up, #scrollbutton-down { /* 6/10/2021 */
    border-top-width: 1px !important;
    border-bottom-width: 0 !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;
}

/* Inactive tabs: Separator line style */

.tabbrowser-tab .tab-background {
    border-right: 1px solid var(--lwt-background-tab-separator-color, rgba(0, 0, 0, .20)) !important;
}
.tabbrowser-tab:first-child .tab-background {
    border-left: 1px solid var(--lwt-background-tab-separator-color, rgba(0, 0, 0, .20)) !important;
}
/* For dark backgrounds */
[brighttext="true"] .tabbrowser-tab .tab-background {
    border-right: 1px solid var(--lwt-background-tab-separator-color, var(--lwt-selected-tab-background-color, rgba(255, 255, 255, .20))) !important;
}
.tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-background {
    border-radius: 0 !important;
}

/* Remove padding between tabs */
.tabbrowser-tab {
    padding-left: 0 !important;
    padding-right: 0 !important;
}

.tabbrowser-tab:is([visuallyselected="true"], [multiselected]) .tab-background {
    box-shadow: -0.5px -2px 0 -0.5px var(--tab-line-color, var(--tabs-border-color, rgba(128,128,142,0.9))), 0 0 4px rgba(128,128,142,0.5) !important;

/* Tweak Options as of 6/12/2021; Generated Mon Jun 14 2021 17:48:55 GMT-0400 (Eastern Daylight Time) */

/* Adjustments for v96+ */
    border-top-width: 2px !important;
    border-bottom: none !important;
    border-left: none !important;
    border-right: none !important;
}