These support forums are for issues with browsing using Firefox. I suggest asking for help on the MDN IRC channel or talking to another developer about this issue.

Does this also happen if you run the code in the Web Console (Firefox/Tools > Web Developer)?

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