Debugging a family computer, I see that it is running Firefox 12. The mozilla.org website is available and working. The https://www.mozilla.org/en-US/firefox/new/ page shows that I am running an old and deprecated firefox and proposes a new download. However, the sumo link to the update doc https://support.mozilla.org/fr/kb/update-firefox-latest-version gives a ssl_error_no_cypher_overlap error without any potential override. This gives a pretty bad user experience.
When configuring SSL, we have to make tradeoffs like this. We try to pick a secure and useful set of ciphers. Unfortunately, we can't get everyone. Firefox 12 on Windows XP is a combination we didn't optimize for. I don't know all the specifics, but I'd imagine that all of the ciphers supported by Fx12/WinXP are considered insecure today. Offering those ciphers would put our other users at risk. Unfortunately, I don't think there is a good solution available to us today.