Usually there's no border line on the bottom side of the active tab. However, if you use an extension that places that tab into a Container, there should be a heavy line color-coded to the relevant container at the bottom of the tab (not completely edge-to-edge). Could that be the issue?

jscher2000 said

Usually there's no border line on the bottom side of the active tab. However, if you use an extension that places that tab into a Container, there should be a heavy line color-coded to the relevant container at the bottom of the tab (not completely edge-to-edge). Could that be the issue?

I don't think so. I have no extensions involving containers, to the best of my knowledge.The two theme extensions where I've seen this issue are Internet Explorer Blue and SebaArt Blue Dream.

Containers are builtin, you do not need an extension.

• https://support.mozilla.org/en-US/kb/containers

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

Here it is, cor-el. The bottom border line for the active tab--this one--should not be there. (This is not too bad with this theme, as the color change is pronounced. The same thing happens however with Internet Blue, which is my preferred theme.)

Hi Odin3, I don't get the line when I test in a new profile (see attached). Could it be something in your userChrome.css file?

jscher2000 said

Hi Odin3, I don't get the line when I test in a new profile

Is my userChrome the culprit? I don't know. I would show you my userChrome, but (a) a screenshot won't display the entire file and (b) copy-and-paste distorts the contents.

Odin3 said

The line never appears from the outset, but only later. And restarting Fx will remove the line, but (unfortunately) only temporarily.

Can you see a pattern to it? Does it ever happen if you use the built-in Light theme?

Is my userChrome the culprit? I don't know. I would show you my userChrome, but (a) a screenshot won't display the entire file and (b) copy-and-paste distorts the contents.

Yes, please don't paste large amounts of CSS here. You can use a site like Pastebin and share the link back here. Below the box where you paste, there a way to specify that it is CSS, which adds syntax coloring to make it easier to read.

https://pastebin.com/

jscher2000 said

Can you see a pattern to it? Does it ever happen if you use the built-in Light theme?

You can use a site like Pastebin and share the link back here.

You have several CSS rules related to tabs/the tab bar and the title bar. Tabs may appear in the "title bar" when you do not enable the optional Menu Bar or Title Bar. But off the top of my head I can't tell what the cumulative effect of those changes is.

If you are familiar with the Browser Toolbox, you could "inspect" the toolbar area to try and determine where the line comes from (for example, is it a border, or a drop-shadow, or something else). https://developer.mozilla.org/docs/Tools/Browser_Toolbox

Thanks, jscher. Currently my preferred theme [Internet Explorer Blue] is running as it should--no bottom border line. If and when the fault reappears, I'll try your suggestion.

Does toggling drag space in customize mode make a difference?

This could also be some border or box shadow added to the Navigation Toolbar and not a tab bar issue.

Thanks, cor-el. For now I'm using SebaArt Blue Dream, which so far does not trigger the subject fault. Once the fault reappears I'll try to give you an answer.