Hi, Please go to Options --> General --> Performance and uncheck the box for Use Recommended Settings and try changing the Process count to 2 and can also go up but note more cpu and ram will be used. You will need to reboot after making any changes.

Then go to Options --> Privacy & Security and down to Permissions. Please ensure that the box for Prevent Accessibility Services from accessing your browser is checked.

Please let us know if this solved your issue or if need further assistance.

No real difference, with both games open CPU > 50% average, RAM ~ 6 GB

Does about:memory shown any detail what takes up this memory?

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
  remove checkmark: [ ] "Use hardware acceleration when available"
• https://support.mozilla.org/en-US/kb/performance-settings

Close and restart Firefox after modifying the setting to make the change effective.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems
• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

These changes only make things worse. First change had already been done. Playing Sudoku, memory usage went from 4 to 9 GB, CPU running 40-50 ^ on all 4 cores. When I added crossword, memory usage went to 12 GB with CPU in 60-80% range. This is affecting my typing this response. I am going to undo the above 2 changes as they seemingly only made things worse.

Hi, Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove your Firefox program files and then reinstall Firefox. This process does not remove your Firefox profile data (such as bookmarks and passwords), since that information is stored in a different location.

To do a clean reinstall of Firefox, please follow these steps: Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from this page) and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (or open the Firefox menu and click the close button ).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

More information about reinstalling Firefox can be found here.

WARNING: Do not use a third party uninstaller as part of this process. Doing so could permanently delete your Firefox profile data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be easily recovered unless they have been backed up to an external device! See Back up and restore information in Firefox profiles.

Also please give your self a scan : https://www.malwarebytes.com/

Please report back to say if this helped you!