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MacOS Text Glitch
(lemmy.blahaj.zone)
A place to discuss the news and latest developments on the open-source browser Firefox
If it's on a Mac, the two things I can think to try are boot without extensions enabled and see if it still happens and flush your caches using a tool like Onyx. Actually, you could also try and redefine what the default sans-serif font it too, just in case that has anything to do with it.
And finally, are you both on a network that might be running through a proxy server first? If so, maybe some protection feature they have turned on is mangling things.
Whatever it is, the smart quotes aren't being represented either in your default sans-serif font or whatever font it's trying to load.