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

More than one process is normal for the current Firefox. http://www.ghacks.net/2016/07/22/multi-process-firefox/

Multi-process Firefox is codenamed "e10s". https://wiki.mozilla.org/Firefox/multiprocess

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

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

http://www.ghacks.net/2016/07/22/multi-process-firefox/

Type about:config<enter> in the address bar. If a warning screen comes up, press the Be Careful button. At the top of the screen is a search bar.

It's now even worse than before.

What have you tried?

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

It browses fine BUT when I try to enter (type) in to a data entry box anywhere it slows down and runs like treacle until I KILL the Tab

DavidH1131 said

It browses fine BUT when I try to enter (type) in to a data entry box anywhere it slows down and runs like treacle until I KILL the Tab

That's some very strange behavior that could be indicative of malware running on your computer, especially since you're typing when it occurs.

Question: Have you run a virus/malware scan recently? If not, I'd suggest downloading Malwarebytes free version and running a scan.

Link: https://www.malwarebytes.com/

Let us know if you find anything interesting after scanning.

Another Suggestion: Start Firefox in safe mode(not related to Windows Safe Mode)

Safe Mode is a special Firefox mode that can be used to troubleshoot and fix problems. Safe Mode temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes) that might be causing problems. By comparing Firefox behavior in normal mode to its behavior in Safe Mode you may be able to pinpoint the cause of the problem.

How to start Firefox in Safe Mode: https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode#w_how-to-start-firefox-in-safe-mode

Does the issue persist in Firefox's Safe Mode?