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

Yes. Still slow.

Start your Computer in safe mode. Then start Firefox. Try Safe websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

ajm115834 said

Yes. Still slow.

You have a bunch of add-ons   -   in Safe Mode they are disabled. Have you re-enabled them , one by one, in order to find the (possible) culprit   ?
And have you read those articles FredMcD gave you the links to   ?

you have adblock plus version 2.9 - there are currently a number of reports that this recent update causes poor performance, please try to replace it with https://addons.mozilla.org/firefox/addon/ublock-origin/ and see if the issue is still there.

Problem solved! Thanks, all.