Do you have a lot of add-ons installed? If so, go to about:profiles and click restart with add-ons disabled.

You can also:

Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

Thank you for your reply. FF Safe Mode in Win8.1 did not help. Pages load very slow. No difference. I tried Internet Explorer and it is just as slow. What's frustrating is that it is perfect (very fast) on the Win10 side.

Do you think a clean install if Firefox is the answer? Bookmarks are backed up.  I can do a "force removal" with Geek uninstaller and reinstall.   

Perhaps the best thing to do is to refresh the OS.

Thank you very much for your help.

On the command line, navigate to the repository that contains the commit you want to amend. Type git commit --amend and press Enter. In your text editor, edit the commit message, and save the commit. You can add a co-author by adding a trailer to the commit.

Not Me.

Hi odatkid, thank you for sharing your browser details. You have multiple extensions that need to filter every page, such as Adblock Plus and Ghostery. Could you experiment with only a few extensions running at a time to see whether they might be interfering with or unnecessarily slowing each other down?

You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions.

Then cast a critical eye over the list on the right side. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Try disabling extensions with duplicate functionality, and anything you can live without for 12 hours, and see what difference it makes.

Thank you for your reply. FF Safe Mode in Win8.1 did not help. Pages load very slow. No difference. I tried Internet Explorer and it is just as slow. What's frustrating is that it is perfect (very fast) on the Win10 side.

How does dual booting work. Is it the same installation of Firefox and same profile, or a different installation and different profile? Presumably different.

Does your Firefox connect through any filtering software, such as Avast, AVG, Bitdefender, ESET, Kaspersky, or Microsoft Family Safety?