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What distro? Try downloading another copy of Firefox, run it from the folder and see if you have the same issue. https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Newest Ubuntu, nope, still the same, I think they just don't have "an option" for no border, so it defaults to white YIKES

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So it's not a browser issue. Mark it resolved and enjoy.  ;-)

If it wasn't a browser issue - it'd display all of those controls without a border. Instead - it defaults to full 255 border, I'm not gonna enjoy white in 2022, this isn't resolved

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LOL Right, it's not resolved. Is it a browser issue or the OS or Desktop Environment? Spin up a VM or live stick to see if you have the same issue.

I don't see what you can't seem to understand in "I have a custom theme that has no borders for inputs & such" Firefox is using the same GTK/system theme, but instead of displaying them without borders, it makes them the ugly default white ones (probably because it doesn't know how to handle border: none or w.e I put in the CSS there). Please stop spamming on here, you've literally said nothing useful in your posts

Brave handles it well, so how exactly is it not a Firefox issue?

Here's the solution for fellow ricers, you're looking for this line of CSS: frame > border, .frame { border: blablabla; }

That's the line affecting Firefox control borders Make sure you're never the guy who commented 3 times on this post with no solution, nobody likes these

Another solution is to dump this browser, uses way too much resources anyway

My DE provides a means of selecting "no border", yours probably does also.