Sometimes Firefox doesn't load fully. Then it usually will show in the Task Manager under processes but not Applications. This article might help: Firefox won't start - find solutions (the "No error message" section).

Other times, it loads off the screen somewhere. In the latter case, you should see a button/tile on the task bar. If you see that, you might be able to resolve the issue as described in this thread: After this last update, I cannot make the window smaller without disapearing.

A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls

Try to disable hardware acceleration in Firefox.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"
• https://hacks.mozilla.org/2010/09/hardware-acceleration/

• https://support.mozilla.org/kb/how-do-i-upgrade-my-graphics-drivers
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes