Only found a script for color line at icon's bottom.

I have seen posts about container colour lines on Reddit, on the firefox or FirefoxCSS sub or both.

I have seen posts about container colour lines on Reddit, on the firefox or FirefoxCSS sub or both.

Been using the popular one/s before FF91. Now rendered ineffective.

In Firefox 91+ you can no longer disable Proton via "browser.proton.enabled = false" on the about:config page. Best is to leave browser.proton.enabled at its default true value as the false setting can cause issues.

This code still works for me.

.tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
	background-color: var(--identity-icon-color) !important;
	margin: 0 0 !important;
}

In Firefox 91+ you can no longer disable Proton via "browser.proton.enabled = false" on the about:config page. Best is to leave browser.proton.enabled at its default true value as the false setting can cause issues. This code still works for me.

.tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
	background-color: var(--identity-icon-color) !important;
	margin: 0 0 !important;
}

Whoa! It worked, though I had to first remove the script displaying color line at the bottom of the icon.

Thanks!