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This might be a caching issue. For me it helped to delete parts of the browsing data(cookies, cached images and files, site permissions) in the settings. As a first step, you could try to delete the site data for the paradox wikis by clicking on the padlock icon next to the url while you have opened a wiki page and then select "clear cookies and site data". Or maybe it helps to force-close the firefox app and start it again
 
The inability to expand article sections is frustrating and hinders the reading experience.
Have you tried deleting the cached files in your browser settings? This has helped several people with that problem. I think the original problem has been fixed, but the browser caches the bugged version of the file.