Can you please tel us what firefox is showing when you trying to browse any website ?

or try these, Check your proxy settings and set it to No Proxy,

Try to run start Firefox in Safe Mode by holding down the Shift/Options key?

https://support.mozilla.org/kb/Safe+Mode

You can also download a offline installer of firefox,

ftp://ftp.mozilla.org/pub/mozilla.org/firefox/releases/32.0.2/win32/en-US/Firefox%20Setup%2032.0.2.exe

Some problems occurs when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again;

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version).
• Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}