User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.122 Safari/537.36

Steps to reproduce:

1. Windows 10, turn on Windows High Contrast Mode (use High Contrast Black, which is the default).

2. Firefox 75.0, open a new browser window.

3. Go to https://en.wikipedia.org/wiki/Main_Page

4. Open a new browser tab in the same window.

5. Again, go to https://en.wikipedia.org/wiki/Main_Page

6. Repeat steps 4 and 5 - opening a new tab and going to Wikipedia - seven times, so that you have 8 tabs open on the same content.

Actual results:

For me, Windows High Contrast Mode was inconsistently supported on page content.

On the first tab, it wasn't supported at all, i.e. the page content had the usual white background, black text, and blue links that one can see without WHCM.

The second to fifth - and seventh - tabs, WHCM High Contrast Black was correctly supported, with black background, white text, and yellow links.

The sixth tab looked like the first, where high contrast was not supported.

The eighth tab seemed to be supporting a different High Contrast Black theme (possibly a Firefox-colors based theme), with black background, white text, and blue links. The blue links do not pass the WCAG AA color contrast requirements on black background. Nor do the purple "visited links".

The point is that Firefox support for Windows High Contrast Mode is unreliable.
Refreshing the page does not make any difference.
I will attach screenshots of the Windows Settings dialog for HCM, and each of the differently-colored tabs.

Expected results:

Every tab should have looked the same. I was expecting black background, white text, yellow links, because that is what the system settings are for Windows High Contrast Black.

This may be similar to https://bugzilla.mozilla.org/show_bug.cgi?id=1445266, except that here, the window trim is ok - it's the content that is not displaying reliably.

Interestingly, if I close Firefox completely, and then start Firefox with Windows High Contrast mode already on, then I do get the expected colors: black background, white text, yellow links, and green "visited links", all correct as shown in my first attached screenshot.
In that snapshot, labelled Windows 10 Settings dialog showing High Contrast Black setting "On", note the color swatches:
white - Text
yellow - Hyperlinks
green - Disabled Text
black - Background

So I think the actual problem is that Firefox is not initializing properly for WHCM if the user turns on the High Contrast setting when Firefox is already open.

Thank you for reporting this. I've moved it to the a11y access component and added the access keyword.

Updating the Accessibility Team's impact assessment to conform with the new triage guidelines. See https://wiki.mozilla.org/Accessibility/Triage for descriptions of these whiteboard flags.

Because this bug's Severity has not been changed from the default since it was filed, and it's Priority is P3 (Backlog,) indicating it has been triaged, the bug's Severity is being updated to S3 (normal.)