Ok, I found the .mozilla and .thunderbird folders.
I moved them to a new location. This allowed both Thunderbird and Firefox to launch without problems. However this also lost all my bookmarks/passwords etc in Firefox and lost all my email and account settings in Thunderbird. aaaargh!!
I deleted the newly created .mozilla and .thunderbird and moved the originals back to the home folder. Remarkably, after doing this both Thunderbird and Firefox were still able to open without killing my machine. And I got all my info/passwords/etc back.
However don't get your hopes up. As soon as I reboot the computer, the freezing problem came back. Now I am back at square 1.
It seems the only way I can use Firefox or Thunderbird is to move the .mozilla and .thunderbird folders, start the apps, exit them, move the hidden folders back, restart the apps... then if I need to reboot the computer for any reason, I have to do the whole process again.
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