You can check the connection settings.

• Tools > Options > Advanced > Network : Connection > Settings
• https://support.mozilla.org/kb/Options+window+-+Advanced+panel

If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/kb/Firefox+cannot+load+websites+but+other+programs+can

MoniB said

The problem started when I got back from travel about 4 days ago. I was gone for two weeks and used my tablet when on travel. Now whenever I try firefox on desktop PC, it runs very slowly and almost all websites will return the following error message: "The requested URL could not be retrieved" - I had to try half a day before I could finally get to the mozilla support page and refresh. That worked for a short time - but the next day, the same problem - so I refreshed again - worked for a short time, and then the same problem. Today - it didn't work at all - It took a half a day to get mozilla support to work again - but no other websites are currently accessible. My desktop uses Window 7

Thanks - that worked - hopefully will continue to work tomorrow