You can try the options listed at Websites look wrong or appear differently than they should

One other thing that I think isn't covered by that article which seems to be affecting more Mac users lately:

Firefox will adapt the appearance of pages to your system theme if it is believed to be a "high contrast" theme. Pages then look much more "black and white."

To try to work around that: set Firefox to strictly follow page colors and see whether that makes any difference. On the Preferences page:

• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Windows: "3-bar" menu button (or Tools menu) > Options
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the search box at the top of the page, type color and Firefox should filter the page down so you can easily find the "Colors" button. Go ahead and click that.

Try changing this setting:

"Override the colors specified by the page with your selections above"

Below that, it usually says

"Only with High Contrast themes"

Click that and change the setting to

"Never"

then click OK to finish the change.

Does that help at all?

Here's a screenshot of that part of the Colors dialog (I'm sure it's prettier on the Mac):

Now that I see the screenshot you posted in your other thread, I think the color setting is the first thing to check.

Advice from JSCHER2000 did the trick! Thanks!