What font is used in both cases ?

You can right-click and select "Inspect" to open the built-in Inspector with this element selected.

• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/open_the_inspector/

You can check in the Rules tab in the right panel in the Inspector what font-family is used for selected text. You can check in the Fonts tab in the right panel in the Inspector what font is actually used because Firefox might be using a different font than specified by the website.

• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/edit_fonts/

So I have checked the built-in inspector and the font used in both cases (same website deepl.com) appears to be Helvetica. Which, after visiting many other websites written in that font confirms that there is indeed a problem with Helvetica. What can be done to solve this problem?

Could be about these bugs:

• 1828474 - Flatpak/Linux: Font rendering completely broken for certain sites since v112
• 1827950 - Text becomes transparent on various sites, e.g. when a webfont [114]

Note that Firefox 112.0.2 has been released to address this issue.

• https://www.mozilla.org/en-US/firefox/112.0.2/releasenotes/

Fixes an issue where Linux users with bitmap fonts installed may have had entire sections of text invisible to them on some sites (bug 1827950).