Don't know what happened to the image, here is a imgur link that shows it non-corrupted. https://i.imgur.com/vyifY0u.png

[v56+] Type about:preferences#general<enter> in the address box. Under fonts and colors, press the Advanced button. Scroll to the bottom, turn on Allow Web Sites To Choose Their Own.

Make sure you haven't enabled a High Contrast theme in the OS settings.

• We have seen reports that this can be caused by a problem with fonts like Roboto or Helvetica that

you have installed locally and that might be corrupted or are missing some variants. Also, make sure you allow pages to choose their own fonts and that you do not block remote (downloadable) fonts.

• https://support.mozilla.org/questions/1325619 Weird font on some website's

• If you use uBlock Origin make sure not to "block remote fonts".

Allow websites to use their own fonts is already checked, also on uBlock. No high contrast theme either. It's weird because Safari displays the page correctly, but not Firefox. Using inspect element it seems that the specific font it's having trouble with is Georgia Bold.

Try this; remove Georgia Bold. Then try the site again. (Sorry, I don't know how.)

Same situation here. (I have uBlock set up correctly, also Firefox’s setting for letting web pages choose their own fonts.)

I found a web page using Roboto but that displays Cyrillic text correctly: https://www.ranker.com/list/famous-poets-from-ukraine/reference

After changing the Wikipedia theme to either Monobook or Timeless, the Cyrillic title displays properly, even though the Timeless theme uses the same Linux Libertine font as the problematic themes Vector legacy 2010, Vector 2022, and MinervaNeue.

There does seem to be something fishy about Firefox’s handling of this Linux Libertine, though. Do you know Font Ninja? It’s an add-on that shows you which fonts are used on the current web page. Now check out the screenshot of the Wikipedia Preferences page, where the text "Preferences" displays properly, but the sample text under the heading "Linux Libertine" in the Font Ninja window is shown in a sans-serif font. Also note the text there: "This is a system font", which is clearly not talking about Linux Libertine!

I add in the analogous screenshot from Russian Wikipedia (using the same Wikpedia theme) just to show what happens to Cyrillic text there.

My theory is that Firefox is replacing Linux Libertine by another font, some ancient fall-back system font that doesn’t include Cyrillic characters. But it does that only in certain cases, as evidenced by the correct display of different Wikipedia themes that use the very same Linux Libertine font. My guess is that, maybe, the problematic themes apply a certain style, like "Semi-Bold", which triggers this behavior, but I don’t know how to check that.

Hope this helps to pinpoint the problem – and solve it ;-)