User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0

Steps to reproduce:

launched firefox after automatic update

Actual results:

The tabs and the menu is painted black on black background, not visible. On tabs only the icon shows. except for the active tab and the hilighted menu item.

Expected results:

Should work like before. The text appears in white and visible if windows color scheme is switched to 'dark'. With the 'light' setting it is wrong. (Personalize -> colors -> Chose your default app mode radio selection).
windows 10 professional 1703 (build 15063.540)

It could be caused by a userContent.css file. Did you create one?
Try removing moz-win-glass or transparent background.

(In reply to Jan Andre Ikenmeyer [:darkspirit] from comment #1)

It could be caused by a userContent.css file. Did you create one?
Try removing moz-win-glass or transparent background.

I did not create such file. I checked location by http://kb.mozillazine.org/index.php?title=UserContent.css&printable=yes and no such file exists at the pointed location.
More info on experimentation:

1. restarting into "safe mode" removes the problem.
2. in normal mode disabling ALL manually on "extensions" tab, does not remove the problem. (I use foxytab, augmented steam, umatrix and ublock origin)
3. in themes, with selecting "light" it works fine. The problem is when it is set to "default".

And as mentioned it started with the last update

Thanks! Please open about:support, click on "Copy Text to clipboard", paste it into a text file and upload it here (Attach File).

You could try to open about:config, set gfx.webrender.force-disabled to true (to disable Firefox' new graphics engine) and restart Firefox, to see if it circumvents the problem.

I couldn't reproduce this bug with Win10 1903. Neither with WebRender or D3D11. Therefore it might only be present in outdated Win10 versions.
Win10 Pro 1703 support end was October 9, 2018.

(see comment 3)

The bug has a release status flag that shows some version of Firefox is affected, thus it will be considered confirmed.