User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/94.0.4606.114 Safari/537.36

Steps to reproduce:

Yesterday, for the first time after Thunderbird installation on a new Win 10 laptop a couple of weeks ago, I selected "Preferences". Scrolling on the page slowed down and (not responding) came up on the title bar. Running Task Manager, I could see Thunderbird was filling up RAM at an alarming rate, reaching 12Gb in two minutes or so. Ending the task and then repeating the process resulted in the same situation. Of the six Thunderbird processes running, it was apparent that the one relating to the current folder under Inbox was the culprit. I deleted this empty folder, ended the task and tried again. Same outcome, with Inbox then being the culprit process. Eventually, I selected troubleshoot mode and the problem disappeared. However, closing Thunderbird and then restarting (which presumably took me out of Troubleshoot Mode) and the problem reappeared.
I have run a full system scan with Bitdefender this morning and no viruses found.

Expected results:

Preferences should have worked normally and not started filling up RAM

If you have language packs, remove them. They are not needed for spelling. If you can't get in to tools > addons, start thunderbird in troubleshoot mode. See Bug 1728744
https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

I've just deleted the English Language pack (retained the French one) and "Preferences" now works perfectly. Many thanks for the prompt resolution.