Have you tried loading Firefox in safe mode to see if the problem goes away?

Re-loaded Firefox in Safe mode and result dramatic - disc use down from 80% to <5% so add-ons must be the problem ! So thank you - but now how do I find which Add-On is the cause ? Roughly Rufus

Thank you again Malvineous - have now chased down each Addon and culprit was ............................................. "iCloud Bookmarks" ! Disable it and HDD light goes off Re-enable it and HDD useage goes up to 60-90%, and carries on like this for hours! (all data from Windows Task Manager)

Very interesting, thanks for following up with the culprit once you found it!

You should contact iCloud support.

You can remove the following files from your Mozilla/Firefox/Profile directory.

content-prefs.sqlite webappsstore.sqlite

After the removing, your Firefox will be more times faster and stops the HDD activty. These files are always getting bigger with firefox operations and slow down the IO operations.

Here is two examples:

12.11.2017. (From my archived profile directory) content-prefs.sqlite : 229376 bytes webappsstore.sqlite : 98304 bytes

22.12.2018. content-prefs.sqlite : 12910592 bytes webappsstore.sqlite : 5701632 bytes

After the removing, the Firefox automatically creates new files of these. Does not cause probs in the operation of Firefox, if you remove these two files.

Be lucky!