Try following the steps on this Firefox help page. I think this might help.
Also, as a sidenote, is your system time correct? The following info is from the Mozilla Article linked.
Internet Security software blocking Firefox
If you were able to load websites until you updated Firefox or until
your Internet security software was updated, reconfigured or a new one
was added, your Internet security software (including firewalls,
antivirus programs, anti-spyware programs, and more) is likely
preventing Firefox from connecting to the Internet.
Note: Some Internet security programs can block Internet access even
when they are in a "disabled" state. In general, you should make sure
your Internet security software is up-to-date and remove Firefox from
your program's list of trusted or recognized programs, then add it
back. For instructions on how to configure some programs, see the
Configure firewalls so that Firefox can access the Internet article.
Firefox connection settings
If you connect to the Internet through a proxy server that is having
connection problems, you will not be able to load websites. To check
your Firefox proxy settings:
Click the menu button New Fx Menu and choose Options. Select the
Advanced panel. Select the Network tab. In the Connection section,
click Settings…. Change your proxy settings: If you don't connect to
the Internet through a proxy (or don't know whether you connect
through a proxy), select No Proxy. If you connect to the Internet
through a proxy, compare Firefox's settings to another browser's (such
as Internet Explorer — see Microsoft's guide to proxy settings). Close
the Connection Settings window. Close the about:preferences page. Any
changes you've made will automatically be saved. If you find that
changes you make to your Firefox connection settings are not
remembered when you restart Firefox, see How to fix preferences that
won't save.
IPv6
Firefox supports IPv6 by default, which may cause connection problems
on certain systems. To disable IPv6 in Firefox:
In the Location bar, type about:config and press Enter. The
about:config "This might void your warranty!" warning page may appear.
Click I'll be careful, I promise! to continue to the about:config
page. In the Search field, type network.dns.disableIPv6. In the list
of preferences, double-click network.dns.disableIPv6 to set its value
to true. DNS Prefetching
Firefox attempts to speed up loading new websites by using DNS
Prefetching, which can cause page load errors with some system
configurations. To disable DNS Prefetching:
In the Location bar, type about:config and press Enter. The
about:config "This might void your warranty!" warning page may appear.
Click I'll be careful, I promise! to continue to the about:config
page. Right-click in the list of preferences, select New, and then
select Boolean. In the Enter the preference name window, enter
network.dns.disablePrefetch and click OK. Select true when prompted to
set the value and click OK. Firefox cannot load certain websites
If you find that Firefox can load some websites but not others, first
clear your Firefox cookies and cache:
Click the menu button New Fx Menu , choose History and select Clear
Recent History.... In the Time Range to clear: drop-down, select
Everything. Click the arrow next to Details to display the list of
items that can be cleared. Select both Cookies and Cache. Click Clear
Now. Check for malware If clearing your cookies and cache did not
enable you to load the websites that did not work in Firefox, you
should check your computer for malware. Certain types of malware are
known to target Firefox and can prevent it from loading various
websites:
If you have an antivirus or Internet security program, update its
detections database and do a full scan of your system. If you still
have problems, follow the instructions in Troubleshoot Firefox issues
caused by malware.
