Firefox Responsive Design Developer Mode - wrong font-weight

As you can see the issue doesn't appear on my end. Did you see the video I attached in my last reply?

I can't locate the cause of issue by just seeing screenshots, I'll have to see the live code to know what's going on.

Does it only happen when you logged in as admin?

What version of Firefox are you using?

I'm not logged into the admin

I'm using 96.0.1, I believe it's up to date.

https://postimg.cc/5YR9jqq4 This screenshot --When the responsive mode is set to 1080p Full HD Television, the fonts are correct.

https://postimg.cc/mzvD8ycd This screenshot -- mode is set to Laptop with HiDPI

https://postimg.cc/XZw4n6Xg This screenshot -- mode is set to iPad Pro

So it only happens when you select iPad Pro?

I did the same, but the fonts are still normal on my end: https://www.screencast.com/t/JpRuMM32AFF

Have you tried on an actual iPad Pro device?

No, it only doesn't happen on the two -- It happens on everything else.

I use Firefox for responsive design work. I don't have this problem with any other site.

Is there a webkit css line that I'm missing? Is there a way to target Firefox specifically? Is there a setting I've missed somehow?

I'm not an expert on the browser, I can't really tell what the cause is.

The font-weight CSS should be supported by all browsers, so I don't think CSS would be the problem.

Does it happen on private window as well?

Can you move the mouse over the heading, right click, inspect.

Then enter font-weight in the filter field, take a screenshot like this:
https://www.screencast.com/t/3cbaqSwemi