hello sasa vilic,

i read your question this is not a bug but please follow the steps.

Clear all cookies, site data and cache

• Click the menu button. and select Options. Preferences.
• Select the Privacy & Security panel and go to the Cookies and Site Data section.
• Click the Clear Data… button. The Clear Data dialog will appear. ...
• Click Clear

restart the firefox and retry.

Thank you!

The same with acid3 (97/100): http://acid3.acidtests.org/

The same with acid3 (97/100): http://acid3.acidtests.org/

After clicking on "this reference rendering" :

(Edge scores 98/100, before clicking on "this reference rendering")

I noticed that my firefox (77.0.1 (64-bit)) doesn't completely render acid2 test correctly. Is this a bug in firefox or in acid test?

Html/css specification was changed once or twice over these years, but the test hasn't been updated. Every modern browser follow the spec.

^:"this reference rendering" shows what the result of the test should be if all tests are completed successfully.
If the result is different than the reference then something is wrong.
Only if the result fully matches the reference image then you score 100%.

^:"this reference rendering" shows what the result of the test should be if all tests are completed successfully

I wanted to show what the colors look like  (the text explains what clicking on that link does .... )