|I don't know of a Firefox setting that could cause this but perhaps the best first step would be to download their latest version and try installing it again. Ideally I would uninstall it first, although if you have any custom settings you had better note them because they will get lost.|
If you still have trouble I'll prowl around Firefox and see if I can find anything useful.
I have seen some suspicions raised about firewall settings causing this (possibly in routers). Here's one of them:
Read all the responses because there are some fixes given. It might be worth you "temporarily" disabling your firewall and then, with Firefox, going to a safe website where you are getting this problem. As implied, some routers have firewalls.
I hope this lot gives you some food for thought.
How to know you are getting old 3:
You start converting your age to hexadecimal