It would not hurt to do a scan for malware.

• Malwarebytes' Anti-Malware - http://www.malwarebytes.org/mbam.php
• SuperAntispyware - http://www.superantispyware.com/
• Spybot Search & Destroy - http://www.safer-networking.org/en/index.html
• AdAware - http://www.lavasoftusa.com/software/adaware/

If these don't find it or can't clear it, post in one of these forums for specialized malware removal help:

• http://www.spywarewarrior.com/index.php
• http://forum.aumha.org/
• http://www.spywareinfoforum.com/
• http://bleepingcomputer.com/

Ok. I'll give it a try. (I already use Malwarebytes)

Nope. That didn't work. The banner is still there.

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 select "No Proxy" if the default "Use the system proxy settings" setting doesn't work.

See "Firefox connection settings":

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

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

Create a new profile as a test to check if your current profile is causing the problems.

See "Basic Troubleshooting: Make a new profile":

• https://support.mozilla.org/kb/Basic+Troubleshooting#w_8-make-a-new-profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" in case there are still problems.

If that new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Really? All that?  :(

Just start with the first steps and work though the list to see if any of those help.

You can start with a new profile to rule out problems caused by your current Firefox profile.