Before reading further, I HIGHLY recommend you follow instructions posted by two previous posters.
However, as a simple and often overlooked possability, please complete IN ORDER: Open IE and set it as your default browser. Uninstall firefox. Restart your computer. Open IE and reset everything to default. Go to mozilla and download the most recent firefox. Install firefox and set as your default browser.
This may not fix the problem, as stated I recommend you follow the previous instructions first as it may be an issue caused by your ISP. However this is simply overlooked a lot of times.
-- Windows 7 Ultimate on custom built system, Windows 10 on under powered laptop. Sophos UTM 9, Ubuntu Server and Windows Server 2008 R2. HyperV Virtualization --
"The hottest places in hell are reserved for those who in a period of moral crisis maintain their neutrality," John F. Kennedy