User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:89.0) Gecko/20100101 Firefox/89.0

Steps to reproduce:

Set theme to Light

Actual results:

There seems to be a color change in today's update to Nightly. Using the Light theme, the URL Bar and the Search Bar now have gray backgrounds when not focused, of exactly the same color as unselected tabs, with only a thin, slightly-lighter gray line between them. The selected tab is likewise only barely brighter than the unselected tabs.

This results in a general blur of indistinguishable grays across the entire navigation area. In particular, finding the currently selected tab is far more difficult, and adds friction to any attempt to switch tabs, since it feels like a blind hunt every single time I need to focus on that area of the browser.

The Default theme has a high contrast between the selected (light) and unselected (dark) tabs. The Dark theme has decent contrast between the selected (darker gray) and unselected (black) tabs.

In addition, many installed themes seem to have lost any distinction in color between selected and unselected tabs, presumably because of a change in element naming leading to their customizations being lost, and being left with only the background or transparent color for all tabs.

Expected results:

The Light theme should retain enough contrast between selected and unselected tabs that they are easy to distinguish. There also needs to be greater contrast between the URL Bar/Search Bar and the rest of the navigation area that they don't look like one solid blob of gray. (The Default theme keeps those two input bars relatively brighter than the surrounding area, so they don't just bleed together.)

Additional measures may need to be taken to ensure custom themes don't get broken by this change.

The Bugbug bot thinks this bug should belong to the 'Firefox::Tabbed Browser' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

I was able to reproduce this issue on latest Nightly version 89.0a1 (2021-03-29)(64-bit) on Windows 10,
I'll set the component to have a starting point from this. Please if not the right component feels free to re rout to the appropriate component.
The severity suggested is S3

same here, 91 on windows 10, brand new profile with proton prefs flipped. it's not an element name change, but a change to the way theme API parameters (converted to CSS variables) are actually used by the internal stylesheets. thank god firefox doesn't have any visually impaired users /s