Any reason you are sticking with Mac 10.9? It's old, insecure and unsupported by Apple.

Start Firefox in <u>Safe Mode</u> to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem. *switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance *do NOT click the "Refresh Firefox" button on the Safe Mode start window *https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode *https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems ---- You can try to modify multi-process settings to see if this has effect. *set dom.ipc.processCount to 1 if it is currently set to a higher value (4) *disable multi-process windows in Firefox You can disable multi-process windows in Firefox by setting these prefs to false on the <b>about:config</b> page. *browser.tabs.remote.autostart = false *browser.tabs.remote.autostart.2 = false You can open the <b>about:config</b> page via the location/address bar. You can accept the warning and click "I'll be careful" to continue. *http://kb.mozillazine.org/about:config *http://www.ghacks.net/2016/07/22/multi-process-firefox/ ---- Try to disable IPv6 (check for other possible causes as well). *https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can