I thought Wikipedia used Arial. Maybe that is only for Windows users??

To research what font the site is using that doesn't look right, try this:

(1) right-click the problem text > Inspect

Firefox should open the Page Inspector panel in the developer tools with the problem element highlighted

(2) on the right side, change the pane to Fonts (you might need to click the little triangle after Layout, Computed, etc. to access the Fonts pane)

Here Firefox should show the font requested by the site and the font on your system being used to satisfy the request. What are you seeing?

here it is. it requested and used "times". i have this font on my system. also this problem occurs on gmail for example (not occuring in other browsers)

Upon closer inspection, Wikipedia has this rule for the main heading (H1):

font-family: 'Linux Libertine','Georgia','Times','Source Serif Pro',serif;

On Windows, Firefox uses Georgia, and if I edit the rule to remove Georgia, Firefox uses Times New Roman, which seems to be automatically used for "Times".

When I look at the detail on your Fonts panel, it doesn't look like anything I've seen before, which is typically just a file or font name. Your Times font file might not be a format that Firefox is able to read properly. However, I don't know anything about font files on Linux. Hopefully another forum volunteer can chime in.