Has it been this way, or did this start recently?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

Thank you Fred,

This has been going on for some time. It is just getting so annoying that I am trying to figure it out.

I restarted Firefox in Safe Mode. So far the fan in my MacBook Pro has not started again (about 5 minutes).

Also, if I open Safari and go to the same websites (like CNN and Facebook) the fan does not run.

It could be the work of one of your add-ons. Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

I have deactivated all add-ons and plug-ins. Still getting the fan.

Hi   !

Would you try this :

3-bar menu => Options => Advanced => General tab => under 'Browsing' :

uncheck ":   'Use hardware acceleration when available'.

Also take a look at these articles, please :

Troubleshoot extensions, themes and hardware acceleration to solve Firefox problems

Upgrade your graphics drivers to use hardware acceleration and WebGL

When multi-process windows in Firefox is enabled then Firefox will use more memory and may be less responsive. Try to disable multi-process windows in Firefox to see if that has effect.

You can disable multi-process windows in Firefox by setting these prefs to false on the about:config page.

• browser.tabs.remote.autostart = false
• browser.tabs.remote.autostart.2 = false

You can open the about:config 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/