On Mac there's an option named "Increase contrast", with the option enabled the icons are hard to distinguish whether we can navigate back or forward. To be precise, there's another option named "Reduce transparency" which is automatically enabled when the "Increase contrast" is on, and it looks like the "Reduce transparency" causes this issue.

I think the "Reduce transparency" setting is likely a red herring.

https://hg.mozilla.org/mozilla-central/rev/22b6e4c68ea6

I tried unsuccessfully to reproduce the bug on MacOs 12 with Firefox version 111.0a1 (2023-01-23) following these steps:

1. Press Cmd+Opt+F5 (to open Accessibility Shortcuts).
2. Check “Increase Contrast” (when you do this, “Reduce Transparency” is automatically checked).
3. Launch Firefox.
4. Open a random page (e.g. https://en.wikipedia.org/wiki/Car).
5. Click on a link.
6. Click on the “Go back one page” button.
7. Notice the “Go back one page” and “Go forward one page” buttons.

Also, I can not notice any difference between the 111.0a1 (2023-01-23) and latest Nightly 113.0a1 (2023-03-29) Firefox versions in which regards these buttons.

The fix can not be confirmed, if we can not reproduce it, so could you please answer a few questions:

1. Do the arrows look like in the attached image and you find them not visible enough?
   If they don’t look like that, could you please offer some more details:
2. What OS version were you using?
3. What Firefox version were you using?
4. What theme were you using in your OS? if any.
5. What theme were you using in Firefox? if any.
6. A picture would help.
7. Any other details you think might be relevant.

I am afraid I am no longer able to reproduce the issue on my macbook. IIRC at that time the OS theme was light one, and the OS version was (is still) 12. I don't use any theme on Firefox.