Hi,

If you have security software (antivirus, firewall etc.) installed, you can try deleting all existing instances of Firefox and its related processes/files in all the different configuration modules/areas of the security software like process monitoring, sub/spawned process control, virtualization, HIPS etc., in addition to the security software's main configuration panel. Instead create new fully allowed/trusted rules for Firefox + its related processes in all the different modules/panels - a genuine/original Firefox installer as well as all the installed EXEs (Application) and DLLs (Application extension) are digitally signed by Mozilla Corporation (right-click > Properties > Digital Signatures). Even otherwise, some security software may also ask again when Firefox and/or its sub processes are started, and you may be able to allow/trust at that time. Please see this.

AV/Firewalls Configuration

If security software interference can be ruled out, please reset Firefox and check.

The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

Did this fix your problems? Please report back to us!

If you would like to save a listing of the add-ons, important modified preferences/settings etc. to quickly replicate the previous profile, please Copy all to clipboard (on the left of the reset button), and paste it into Office Excel, or OpenOffice/LibreOffice Calc, or notepad. However, it would be a good idea to check for the existence of the issue immediately after the reset.

Spybot S&D and Windows Security Essentials are installed and active, but I cannot find any assocations with Firefox. Windows Defender is installed but has not been running--does not appear relevant. Resetting FF settings to factory defaults makes no difference. I'll need to get back to this Sunday or Monday since I am scheduled to leave for the weekend. Thanks for the efforts and suggestions thus far. Problem appears to be limited to one site--AFAIK.

Is it when you you're signed in, and inside a custom page?

Please create a screenshot of the problem you're seeing by following the guide mentioned at How do I create a screenshot of my problem?.

Once you've done this, attach the saved screenshot file to your forum post by clicking the Browse... button below the Post your reply box. This will help us to visualize the problem.

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

• Do not click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

If you have increased the minimum font size then try the default setting "none" in case the current setting is causing problems.

• Tools > Options > Content : Fonts & Colors > Advanced > Minimum Font Size (none)

Make sure that you allow websites to choose their fonts.

• Tools > Options > Content : Fonts & Colors > Advanced > [X] "Allow pages to choose their own fonts, instead of my selections above"

• https://support.mozilla.org/kb/Changing+fonts+and+colors

Try to boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.