check this:

• see if there are updates for your graphics drive drivers

https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)

https://forums.adobe.com/message/4468493#TemporaryWorkaround

• disable hardware acceleration in the Flash plugin

https://forums.adobe.com/thread/891337 See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

Make sure your Java (Not JavaScript) is up to date Verify Java and Find Out-of-Date Versions {web link}

Version 8 Update 60 Release date August 18, 2015

Works for me on Linux. There is an iframe with source www.google.com/recaptcha/api2/

Do you see anything if you right-click in the space where the captcha should be and open the Inspector via Inspect Element?

• https://developer.mozilla.org/Tools/Page_Inspector

You can also check the Web Console for errors.

• https://developer.mozilla.org/Tools/Web_Console

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

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

Make sure that you do not block cookies for the Google site and for the openmailbox site.

You can inspect and manage permissions for the domain in the currently selected tab via these steps:

• Click the "Site Identity Button" (globe/padlock) on the location/address bar
• Click "More Information" to open "Tools > Page Info" with the Security tab selected

Go to the Permissions tab (Tools > Page Info > Permissions) to check the permissions for the domain in the currently selected tab.