Hi Jeff_M:
That might indicate you have corrupted data in your Firefox profile.
If you’d like to continue troubleshooting go to Help | More Troubleshooting Information (or type about:support in your address bar) in Firefox and look for Profile Folder under Application Basics. Make note of the hidden folder location and name (a random string of 8 characters) of your current default Firefox profile - it should be something like C:\Users\<yourusername>\AppData\Roaming\Mozilla\Firefox\Profiles\xxxxxxxx.default-release.
As an extra precaution, click the Open Folder button and back up that profile (i.e., the entire xxxxxxxx.default folder) to a safe location like a removable USB stick as instructed at Back up and restore information in Firefox profiles. I would also create a fresh backup of your bookmarks as a separate .json file (Bookmarks | Manage Bookmarks | Import and Backup | Backup) to the same location.
Then create a new Firefox profile (Step 6 of Troubleshoot and diagnose Firefox problems) and add the uBlock Origin extension for extra content blocking, but do not import your bookmarks or any other data from your old profile or add any other browser extensions. Use Firefox for an hour or two with all its default settings and then manually clear your cache from within Firefox (Step 2 of that article - i.e., History | Clear Recent History | Everything) and see if that speeds up your cleaning time. ***
If that clears your cache quickly use your browser for another hour or two, close Firefox, and then clear your cache again using CCleaner and see how long it takes.
If your cache clearing is still slow in either Firefox or CCleaner post back and let us know what antivirus and/or other security software (e.g., Malwarebytes Premium) you use that loads at Windows startup.
*** NOTE: If the problem persists in your new Firefox profile you can switch back to your old Firefox profile using the built-in Profile Manager (about:profiles). Each Firefox profile stores its own set of bookmarks, passwords, extensions, and settings so nothing should need to be restored once you’ve switched back to your old profile.
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Dell Inspiron 15 5584 * 64-bit Win 10 Pro v22H2 build 19045.6218 * Firefox v142.0.1 * Microsoft Defender v4.18.25070.5-1.1.25070.4 * Malwarebytes Premium v5.3.7.209-139.0.5357 * Macrium Reflect Free v8.0.7783 * CCleaner Free Portable v6.39.11548 * HDCleaner Portable 64-Bit v2.091