User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:82.0) Gecko/20100101 Firefox/82.0

Steps to reproduce:

Install Firefox 82 on elementary OS 5.1.7. Hera and enable the dark theme, the light theme and the Alpenglow theme.

Actual results:

Looking at the default theme without the title bar disabled - perfect round window corners. When enabling the other themes - no round window corners.

Expected results:

Round window corners enabled in all themes, not just the default theme.

(In reply to mikolov73 from comment #0)

Created attachment 9183527 [details]
Screenshot-dark-theme.png

User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:82.0) Gecko/20100101 Firefox/82.0

Steps to reproduce:

Install Firefox 82 on elementary OS 5.1.7. Hera and enable the dark theme, the light theme and the Alpenglow theme.

Actual results:

Looking at the default theme with the title bar disabled - perfect round window corners. When enabling the other themes - no round window corners.

Expected results:

Round window corners enabled in all themes, not just the default theme.

(In reply to mikolov73 from comment #0)

Created attachment 9183527 [details]
Screenshot-dark-theme.png

User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:82.0) Gecko/20100101 Firefox/82.0

Steps to reproduce:

Install Firefox 82 on elementary OS 5.1.7. Hera and enable the dark theme, the light theme and the Alpenglow theme.

Actual results:

Looking at the default theme without the title bar disabled - perfect round window corners. When enabling the other themes - no round window corners.

Expected results:

Round window corners enabled in all themes, not just the default theme.

(In reply to mikolov73 from comment #0)

... Looking at the default theme without the title bar disabled ...

Sorry, I meant to write with the title bar disabled, not without.

This is because the build-in themes are not Gtk themes so the titlebar is not styled by Gtk.
The rounded corners are property of Gtk theme and are present only if the actual Gtk theme includes it.

Now I'm forced to use Elementary Tweaks to have a dark theme with rounded corners for unpainted apps like Firefox. Elementary Tweaks gives me a dark theme while the default theme is enabled. The dark theme I wrote myself in userChrome. Firefox now has rounded corners in the dark theme of the system. I hope this is helpful for some.

It has been a year now and I just updated to firefox 88 but the bug still occurs

We're close.