Had you tried v65.0.2 ?

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Thank you!

This white bar. I think it's because v64 didn't have it and now that v65 has it as a feature and it's not the same as the other tabs it low-key bothers me.

FredMcD said

Had you tried v65.0.2 ?

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

It is V65.0.2 and in safe mode it is still there.

Link so others can see if it happens on their side?

It's only on the selected tab you're viewing. Like it's a tab feature. So it doesn't matter the website.

That line is blue for me, but it could be reflecting your Windows 10 system theme. Could you try switching from "Default" to "Dark" and see whether that makes any difference. This article describes those built-in themes: Built-in themes in Firefox - alternative to complete themes.

Tried several themes and the bar was white for all but 1. Do you have it on your firefox?

I have the blue bar on the active tab, starting in Firefox 57 through now. I've never seen it turn white, although my testing with "High Contrast" themes has been very limited.

Interesting because they didn't show up for me until v65.